Photomask, method for producing the same, and method for forming pattern using the photomask

ABSTRACT

A photomask includes a semi-light-shielding portion having a light-shielding property, a light-transmitting portion surrounded by the semi-light-shielding portion and a peripheral portion positioned in a periphery of the light-transmitting portion on a transparent substrate. The semi-light-shielding portion and the light-transmitting portion transmit the exposure light in the same phase each other, whereas the peripheral portion transmits the exposure light in a phase opposite to that of the light-transmitting portion. A phase shift film that transmits the exposure light in a phase opposite to that of the peripheral portion is formed on the transparent substrate-in the semi-light-shielding portion formation region.

RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 11/312,349,filed Dec. 21, 2005 now U.S. Pat. No. 7,144,684, which is a divisionalof U.S. patent application Ser. No. 10/424,722, filed Apr. 29, 2003 (nowU.S. Pat. No. 7,001,694), which claims priority of Japanese applicationSerial No. 2002-128021, filed Apr. 30, 2002, and the contents of whichare hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a photomask for forming a fine patternused for producing a semiconductor integrated circuit device, a methodfor producing the same and a method for forming a pattern using thephotomask.

In recent years, it is increasingly necessary to miniaturize circuitpatterns for high integration of a large-scale integrated circuit device(hereinafter, referred to as “LSI”) that can be realized withsemiconductors. As a result, a reduction of the width of a line forwiring patterns constituting a circuit or miniaturization of contacthole patterns (hereinafter, referred to as “contact patterns”) thatconnect between layered wirings formed via insulating layers have becomevery important.

Hereinafter, miniaturization of wiring patterns with a recentlight-exposure system will be described by taking the case of using apositive resist process as an example. In a positive resist process, aline pattern refers to a line-shaped resist film (resist pattern) thatare left, corresponding to a non-exposed region of a resist by exposurewith a photomask and subsequent development. A space pattern refers to aportion from which a resist is removed (resist-removed pattern)corresponding to an exposed region of a resist. A contact pattern refersto a hole-like resist-removed portion and can be regarded as a smallspace pattern of the space patterns. When using a negative resistprocess instead of a positive resist process, the definition of the linepattern and the definition of the space pattern are replaced by eachother.

In general, for miniaturization of wiring patterns, a method for forminga fine line pattern with oblique incident light exposure (off-axisillumination) called super resolution exposure has been used. Thismethod is an excellent method for miniaturization of a resist patterncorresponding to a non-exposed region of a resist, and also has aneffect of improving the depth of focus of dense patterns that arearranged periodically. However, this oblique incident exposure methodhas little effect on miniaturization of isolated resist-removedportions, and on the contrary, this method deteriorates the contrast ofimages (optical images) and the depth of focus. Therefore, the obliqueincident exposure method is positively used to form patternscharacterized in that the size of the resist-removed portion is largerthan the size of a resist pattern, for example, to form gate patterns.

On the other hand, to form a micro resist-removed portion that isisolated such as a small contact pattern, it is known that it is usefulto use a small light source having a low coherence degree that containsno oblique incident component. In this case, it is more useful to use ahalf-tone phase-shifting mask (see, for example, Japanese Laid-OpenPatent Publication No. 9-90601). In the half-tone phase-shifting mask, aphase sifter that has a very low transmittance of about 3 to 6% withrespect to exposure light and causes phase inversion of 180 degrees withrespect to light transmitted through an opening, instead of a completelight-shielding portion, is provided as a mask pattern surrounding alight-transmitting portion (opening) corresponding to a contact pattern.

In this specification, a transmittance is represented by an effectivetransmittance when the transmittance of a transparent substrate is takenas 100%, unless otherwise specified. Moreover, “complete light-shieldingfilm (complete light-shielding portion) refers to a light-shielding film(light-shielding portion) having an effective transmittance of smallerthan 1%.

Hereinafter, the principle of the method for forming patterns using ahalf-tone phase-shifting mask will be described with reference to FIGS.27A to 27G.

FIG. 27A is a plan view of a photomask in which an opening correspondingto a contact pattern is provided in a chromium film serving as acomplete light-shielding portion provided on the surface of the mask.FIG. 27B shows the amplitude intensity corresponding to line AA′ oflight transmitted through the photomask shown in FIG. 27A. FIG. 27C is aplan view of a photomask in which a chromium film corresponding to acontact pattern as a complete light-shielding portion is provided in aphase shifter provided on the surface of the mask. FIG. 27D shows theamplitude intensity corresponding to line AA′ of light transmittedthrough the photomask shown in FIG. 27C. FIG. 27E is a plan view of aphotomask in which an opening corresponding to a contact pattern isprovided in a phase shifter provided on the surface of the mask (i.e., ahalf-tone phase-shifting mask). FIGS. 27F and 28G show the amplitudeintensity and the light intensity corresponding to line AA′ of lighttransmitted through the photomask shown in FIG. 27E, respectively.

As shown in FIGS. 27B, 27D, and 27F, the amplitude intensity of lighttransmitted through the half-tone phase-shifting mask shown in FIG. 27Eis equal to the sum of the amplitude intensities of lights transmittedthrough the photomasks shown in FIGS. 27A and 27C. That is to say, inthe half-tone phase-shifting mask shown in FIG. 27E, the phase shifterserving as a light-shielding portion is configured so as to not onlytransmit light at a low transmittance, but also provide an optical pathdifference (phase difference) of 180 degrees with respect to the lighttransmitted through the opening to the light transmitted through thisphase shifter. Therefore, as shown in FIGS. 27B and 27D, the lighttransmitted through the phase shifter has an amplitude intensity with aphase opposite to that of the light transmitted through the opening.Thus, if the amplitude intensity distribution shown in FIG. 27B and theamplitude intensity distribution shown in FIG. 27D are synthesized, aphase boundary in which the amplitude intensity is turned to 0 by aphase change is generated, as shown in FIG. 27F. As a result, as shownin FIG. 27G, in the end of the opening that is the phase boundary(hereinafter, referred to as a “phase end”), the light intensity, whichis represented by a square of the amplitude intensity, becomes 0, and asignificantly dark portion is formed. Accordingly, in an image of thelight transmitted through the half-tone phase-shifting mask shown inFIG. 27E, strong contrast is realized in the vicinity of the opening.However, the following should be noted: This improvement of the contrastoccurs with respect to light vertically incident to the mask, morespecifically, that is, light incident to the mask from a small lightsource region having a low coherence degree. However, the contrast isnot improved even in the vicinity of the opening (in the vicinity of thephase boundary in which a phase change occurs) with respect to obliqueincident exposure light, for example, exposure called annularillumination in which a vertical incident component (illuminationcomponent from the center of a light source (the normal direction of themask) is removed. Furthermore, there is another disadvantage in thatcompared with the case where exposure is performed with a small lightsource having a low coherence degree, the depth of focus is lower in thecase where oblique incident exposure is performed.

As described above, in order to form a fine resist-removed pattern suchas a contact pattern using a positive resist process, it was necessaryto perform exposure with a small light source having a coherence degreeof about 0.5 or less, which provides illumination only with verticalincident components, in combination with a half-tone phase-shiftingmask. This method was very useful to form fine and isolated contactpatterns.

There is a recent tendency associated with a high degree of integrationof recent semiconductor devices that densely arranged patterns as wellas isolated patterns are also required not only for wiring patterns butalso contact patterns. In order to realize a high depth of focus whenforming densely arranged contact patterns, oblique incident exposure isuseful as in the case of the densely arranged wiring patterns.

Furthermore, in recent years, also when forming wiring patterns, inaddition to miniaturization of line patterns serving as wiring patterns,there is an increasing demand for miniaturization of space patternsbetween wirings. As in the case of the isolated contact patterns, it isuseful to use a light source having a low coherence degree incombination with a half-tone phase-shifting mask in order to form smallisolated space patterns between wirings.

That is to say, although oblique incident exposure is essential to formhigh density wiring patterns and high density contact patterns, thecontrast and the depth of focus of isolated contact patterns andisolated space patterns between wirings are significantly deterioratedwhen oblique incident exposure is performed. The contrast and the depthof focus are deteriorated even more significantly when a half-tonephase-shifting mask is used to improve the resolution.

On the other hand, when a small light source having a low coherencedegree is used to form small isolated contact patterns and smallisolated space patterns between wirings, it becomes difficult to formhigh density patterns or small line patterns.

Therefore, the optimal illumination conditions with respect to smallisolated space patterns and the optical illumination conditions withrespect to densely arranged patterns or small line patterns have acontradictory relationship. Therefore, in order to form small resistpatterns and small isolated resist-removed patterns at the same time, alight source having a medium coherence degree (about 0.5 to 0.6) is usedfor a trade-off between the effect of vertical incident components froma light source and the effect of oblique incident components from alight source. However, in this case, both the effect of verticalincident components and the effect of oblique incident components arecanceled, so that it is difficult to realize further high integration ofsemiconductor devices by miniaturizing isolated line patterns or denselyarranged patterns and isolated space patterns at the same time.

SUMMARY OF THE INVENTION

Therefore, with the foregoing in mind, it is an object of the presentinvention to miniaturize isolated space patterns and isolated linepatterns or dense patterns at the same time.

In order to achieve the above object, a photomask of the presentinvention includes a semi-light-shielding portion having alight-shielding property with respect to exposure light, alight-transmitting portion surrounded by the semi-light-shieldingportion and having a light-transmitting property with respect toexposure light and a peripheral portion surrounded by thesemi-light-shielding portion and positioned in a periphery of thelight-transmitting portion on a transparent substrate. Thesemi-light-shielding portion and the light-transmitting portion transmitthe exposure light in the same phase each other. The peripheral portiontransmits the exposure light in a phase opposite to that of thesemi-light-shielding portion and the light-transmitting portion. A phaseshift film that has a transmittance allowing the exposure light to betransmitted partially and transmits the exposure light in a phaseopposite to that of the peripheral portion is formed on the transparentsubstrate in a formation region for the semi-light-shielding portionformation region.

According to the photomask of the present invention, the peripheralportion that transmits exposure light in a phase opposite to that of thelight-transmitting portion is sandwiched by the light-transmittingportion and the semi-light-shielding portion that transmits exposurelight in the same phase as that of the light-transmitting portion. As aresult, the contrast in the light intensity distribution between thelight-transmitting portion and the peripheral portion can be enhanced bymutual interference between the light transmitted through thelight-transmitting portion and the light transmitted through theperipheral portion. This contrast enhancement effect also can beobtained when a fine isolated resist-removed portion (i.e., a fineisolated space pattern corresponding to the light-transmitting portion)is formed with oblique incident exposure (off-axis illumination), forexample, in the positive resist process. That is to say, a combinationof the photomask of the present invention and oblique incident exposurecan miniaturize isolated space patterns and isolated line patterns ordense patterns at the same time.

In this specification, “having light-transmitting properties withrespect to exposure” means having a transmittance that allows a resistto be exposed, and “having light-shielding properties with respect toexposure” means having a transmittance that does not allow a resist tobe exposed. The “same phase” means a phase difference of (−30+360×n)degrees or more and (30+360×n) degrees or less, (where n=an integer),and the “opposite phase” means a phase difference of (150+360×n) degreesor more and (210+360×n) degrees or less.

In the photomask of the present invention, the transparent substrate ina formation region for the light-transmitting portion may be dug down soas to have a thickness that transmits the exposure light in a phaseopposite to that of the peripheral portion. In other words, thelight-transmitting portion may be a substrate-dug portion serving as ahigh transmittance phase shifter.

The photomask of the present invention, the surface of the transparentsubstrate in a formation region for the peripheral portion may beexposed.

The photomask of the present invention, the phase shift film may be ametal-containing oxide film.

The photomask of the present invention, it is preferable that the phaseshift film includes a transmittance adjusting film having atransmittance lower than that of the transparent substrate with respectto the exposure light, and a phase adjusting film that is formed on thetransmittance adjusting film and transmits the exposure light in a phaseopposite to that of the peripheral portion.

With this feature, a combination of a desired phase change and a desiredtransmittance can be selected arbitrarily for the phase shift film.Moreover, a combination of the material of the transmittance adjustingfilm and the material of the phase adjusting film makes it possible toimprove the selection ratio at etching for processing the phase shiftfilm.

When the phase shift film has a transmittance adjusting film and a phaseadjusting film, the transmittance adjusting film may be a thin film madeof a metal or a metal alloy. The transmittance adjusting film may have athickness of 30 nm or less.

When the phase shift film has a transmittance adjusting film and a phaseadjusting film, the phase adjusting film may be an oxide film.

When the phase shift film has a transmittance adjusting film and a phaseadjusting film, it is preferable that the peripheral portion is disposedapart from the light-transmitting portion by a predetermined distance,and only the transmittance adjusting film of the phase shift film isformed between the peripheral portion and the light-transmittingportion. With this feature, the average of the transmittance of theperipheral portion and the transmittance of a portion in which only thetransmittance adjusting film is formed between the peripheral portionand the light-transmitting portion (hereinafter, referred to as “phaseadjusting film removed portion) becomes smaller than the transmittanceof the peripheral portion. That is to say, the transmittance (effectivetransmittance) of the peripheral portion including the phase adjustingfilm removed portion is smaller than 1, so that a margin for sizecontrol of the peripheral portion can be increased. Furthermore, whenthe transmittance adjusting film is made of a single layered thin film,the light transmitted through the peripheral portion has substantiallythe same phase as that of the light transmitted through the phaseadjusting film removed portion. In this case, compared with the casewhere a transmittance adjusting film in a multilayered structure isused, the peeling of the transmittance adjusting film is prevented whenthe transmittance adjusting film having a small width is formed betweenthe peripheral portion and the light-transmitting portion.

In the photomask of the present invention, the peripheral portion may bedisposed so as to be in contact with the light-transmitting portion ormay be disposed apart from the light-transmitting portion by apredetermined distance.

In the photomask of the present invention, it is preferable that thephase shift film includes a transmittance adjusting film having atransmittance lower than that of the transparent substrate with respectto the exposure light, a phase adjusting film that is formed on thetransmittance adjusting film and transmits the exposure light in a phaseopposite to that of the peripheral portion, and that the transmittanceadjusting film is also formed on the transparent substrate in aformation region for the peripheral portion.

With this feature, a combination of a desired phase change and a desiredtransmittance can be selected arbitrarily for the phase shift film.Moreover, a combination of the material of the transmittance adjustingfilm and the material of the phase adjusting film makes it possible toimprove the selection ratio at etching for processing the phase shiftfilm. Furthermore, since only the transmittance adjusting film is formedon the transparent substrate in the formation region for the peripheralportion, the transmittance of the peripheral portion is lower than thatof the transparent substrate, and therefore the peripheral portionserves as a transmittance adjusting portion. That is to say, thetransmittance of the peripheral portion can be adjusted to a desiredvalue by the transmittance adjusting film. Therefore, it is avoided thatthe transmittance of the peripheral portion is the highest on thephotomask, so that the degree of miniaturization required for theperipheral portion can be reduced. In other words, the problem that theupper limit of the size of the peripheral portion, i.e., the opening inthe outline enhancement mask is small, which makes it difficult toproduce a photomask, can be prevented.

In this case, the transmittance adjusting film may be made of a metal ora metal alloy and transmits the exposure light in the same phase as thatof peripheral portion. In this case, the transmittance adjusting filmmay have a thickness of 30 nm or less.

In this case, the phase adjusting film may be an oxide film.

Furthermore, in this case, the peripheral portion may be disposed so asto be in contact with the light-transmitting portion or may be disposedapart from the light-transmitting portion by a predetermined distance.

In the photomask of the present invention, it is preferable that thephase shift film includes a phase adjusting film that transmits theexposure light in a phase opposite to that of the peripheral portion anda transmittance adjusting film that is formed on the phase adjustingfilm and has a transmittance lower than that of the transparentsubstrate with respect to the exposure light, the phase adjusting filmis also formed on the transparent substrate in a formation region forthe light-transmitting portion, and the surface of the transparentsubstrate in a formation region for the peripheral portion is exposed.

With this feature, a combination of a desired phase change and a desiredtransmittance can be selected arbitrarily for the phase shift film.Moreover, a combination of the material of the transmittance adjustingfilm and the material of the phase adjusting film makes it possible toimprove the selection ratio at etching for processing the phase shiftfilm.

In this case, the transmittance adjusting film may be a thin film thatis made of a metal or a metal alloy and transmits the exposure light inthe same phase as that of peripheral portion. In this case, thetransmittance adjusting film may have a thickness of 30 nm or less.

In this case, the phase adjusting film may be an oxide film.

Furthermore, in this case, the peripheral portion may be disposed so asto be in contact with the light-transmitting portion or may be disposedapart from the light-transmitting portion by a predetermined distance.

In the photomask of the present invention, it is preferable that thetransmittance with respect to the exposure light of the phase shift filmis 6% or more and 15% or less.

With this feature, the contrast enhancement effect can be obtainedreliably while preventing a reduction in thickness of the resist film inpattern formation.

A method for forming a pattern of the present invention uses thephotomask of the present invention and includes the steps of forming aresist film on a substrate; irradiating the resist film with theexposure light via the photomask, and developing the resist filmirradiated with the exposure light so as to pattern the resist film.

According to the method for forming a pattern of the present invention,the same effects as those provided by the photomask of the presentinvention can be obtained. Those effects can be obtained by usingoff-axis illumination (oblique incident exposure) in the step ofirradiating the resist film with the exposure light.

A first method for producing a photomask of the present invention is amethod for producing a photomask including a semi-light-shieldingportion having a light-shielding property with respect to exposurelight, a light-transmitting portion surrounded by thesemi-light-shielding portion and having a light-transmitting property,and a peripheral portion surrounded by the semi-light-shielding portionand positioned in a periphery of the light-transmitting portion on atransparent substrate. More specifically, the method includes a firststep of forming a phase shift film that has a transmittance allowing theexposure light to be transmitted partially and transmits the exposurelight in a phase opposite to that of the peripheral portion on thetransparent substrate in the semi-light-shielding portion formationregion, and a second step of digging down the transparent substrate inthe light-transmitting portion formation region so as to have athickness that transmits the exposure light in a phase opposite to thatof the peripheral portion after the first step.

According to the first method for producing a photomask, after the phaseshift film that transmits exposure light partially in an opposite phaseis formed on the transparent substrate in the light-shielding portionformation region, the transparent substrate in the light-transmittingportion formation region is dug down so as to have a thickness thattransmits the exposure light in the opposite phase. Therefore, theperipheral portion that transmits exposure light in a phase opposite tothat of the light-transmitting portion is sandwiched by thelight-transmitting portion serving as a high transmittance phase shifterand the semi-light-shielding portion serving as a low transmittancephase shifter that transmits exposure light in the same phase as that ofthe light-transmitting portion. Consequently, the contrast in the lightintensity distribution between the light-transmitting portion and theperipheral portion can be enhanced by mutual interference between thelight transmitted through the light-transmitting portion and the lighttransmitted through the peripheral portion. This contrast enhancementeffect also can be obtained when a fine isolated resist-removed portion(i.e., a fine isolated space pattern corresponding to alight-transmitting portion) is formed with oblique incident exposure,for example, in the positive resist process. That is to say, obliqueincident exposure can miniaturize isolated space patterns and isolatedline patterns or dense patterns at the same time.

In the first method for producing a photomask, it is preferable that thefirst step includes the step of removing the phase shift film in thelight-transmitting portion formation region and the peripheral portionformation region after the phase shift film is formed on the entiresurface of the transparent substrate.

With this feature, after the phase shift film is formed on thetransparent substrate, the phase shift film and the transparentsubstrate are etched selectively, and therefore a mask pattern with anyshape can be easily realized that has the semi-light-shielding portionserving as a low transmittance phase shifter and the peripheral portion,and a light-transmitting portion with any shape can be easily realizedthat serves as a high transmittance phase shifter. Furthermore, when thelight-transmitting portion and the peripheral portion are apart, inother words, when the phase shift film is left between thelight-transmitting portion and the peripheral portion, using the phaseshift film patterned in the first step as a mask, the transparentsubstrate can be etched in a self-alignment manner in the second step.Therefore, photomask process can be performed precisely.

In the first method for producing a photomask, it is preferable that thefirst step includes the step of removing the phase shift film in theperipheral portion formation region after the phase shift film is formedon the entire surface of the transparent substrate, and the second stepincludes the step of removing the phase shift film in thelight-transmitting portion formation region before digging down thetransparent substrate in the light-transmitting portion formationregion.

With this feature, after the phase shift film is formed on thetransparent substrate, the phase shift film and the transparentsubstrate are etched selectively, and therefore a mask pattern with anyshape can be easily realized that has the semi-light-shielding portionserving as a low transmittance phase shifter and the peripheral portion,and a light-transmitting portion with any shape can be easily realizedthat serves as a high transmittance phase shifter. Furthermore, the stepof removing the phase shift film in the peripheral portion formationregion and the step of removing the phase shift film in thelight-transmitting portion formation region can be performed separately,so that when the light-transmitting portion formation region and theperipheral portion formation region are apart by a small distance, thatis, when the phase shift film having a small width is left between thelight-transmitting portion and the peripheral portion, a margin forphotomask processing can be increased.

In the first method for producing a photomask, it is preferable that thephase shift film includes a transmittance adjusting film having atransmittance lower than that of the transparent substrate with respectto the exposure light, and a phase adjusting film that is formed on thetransmittance adjusting film and transmits the exposure light in a phaseopposite to that of the peripheral portion.

With this feature, a combination of a desired phase change and a desiredtransmittance can be selected arbitrarily for the phase shift film.Moreover, a combination of the material of the transmittance adjustingfilm and the material of the phase adjusting film makes it possible toimprove the selection ratio at etching for processing the phase shiftfilm.

In the first method for producing a photomask, it is preferable that thefirst step includes the step of forming a transmittance adjusting filmhaving a transmittance lower than that of the transparent substrate withrespect to the exposure light and a phase adjusting film that transmitsthe exposure light in a phase opposite to that of the peripheral portionsequentially on the entire surface of the transparent substrate, andthen removing the phase adjusting film in the light-transmitting portionformation region and the peripheral portion formation region, so thatthe phase shift film including the transmittance adjusting film and thephase adjusting film is formed on the transparent substrate in thesemi-light-transmitting portion formation region, and the second stepincludes the step of removing the transmittance adjusting film in thelight-shielding portion formation region before digging down thetransparent substrate in the light-transmitting portion formationregion.

With this feature, since the transmittance adjusting film is formed onthe transparent substrate in the formation region for the peripheralportion, the transmittance of the peripheral portion is lower than thatof the transparent substrate, and therefore the peripheral portionserves as a transmittance adjusting portion. That is to say, thetransmittance of the peripheral portion can be adjusted to a desiredvalue by the transmittance adjusting film. Therefore, it is avoided thatthe transmittance of the peripheral portion is the highest on thephotomask, so that the degree of miniaturization required for theperipheral portion can be reduced. In other words, the problem that theupper limit of the size of the peripheral portion, i.e., the opening inthe enhancement mask is small, which makes it difficult to produce aphotomask, can be prevented. Furthermore, after the transmittanceadjusting film and the phase adjusting film are formed sequentially onthe transparent substrate, the phase adjusting film, the transmittanceadjusting film and the transparent substrate are etched, and therefore amask pattern with any shape can be easily realized that has thesemi-light-shielding portion serving as a low transmittance phaseshifter and the peripheral portion serving as a transmittance adjustingportion, and a light-transmitting portion with any shape can be easilyrealized that serves as a high transmittance phase shifter. Furthermore,when the light-transmitting portion and the peripheral portion areapart, in other words, when the phase adjusting film is left between thelight-transmitting portion and the peripheral portion, using thepatterned phase adjusting film as a mask, the transparent substrate canbe etched in a self-alignment manner. Therefore, photomask process canbe performed precisely.

In the first method for producing a photomask, it is preferable that thefirst step includes the step of forming a transmittance adjusting filmhaving a transmittance lower than that of the transparent substrate withrespect to the exposure light and a phase adjusting film that transmitsthe exposure light in a phase opposite to that of the peripheral portionsequentially on the entire surface of the transparent substrate, andthen removing the phase adjusting film in the peripheral portionformation region, so that the phase shift film including thetransmittance adjusting film and the phase adjusting film is formed onthe transparent substrate in the semi-light-shielding portion formationregion, and the second step includes the step of sequentially removingthe phase adjusting film and the transmittance adjusting film in thelight-transmitting portion formation region before digging down thetransparent substrate in the light-transmitting portion formationregion.

With this feature, since the transmittance adjusting film is formed onthe transparent substrate in the formation region for the peripheralportion, the transmittance of the peripheral portion is lower than thatof the transparent substrate, and therefore the peripheral portionserves as a transmittance adjusting portion. That is to say, thetransmittance of the peripheral portion can be adjusted to a desiredvalue by the transmittance adjusting film. Therefore, it is avoided thatthe transmittance of the peripheral portion is the highest on thephotomask, so that the degree of miniaturization required for theperipheral portion can be reduced. In other words, the problem that theupper limit of the size of the opening in the outline enhancement maskis small, which makes it difficult to produce a photomask, can beprevented. Furthermore, after the transmittance adjusting film and thephase adjusting film are formed sequentially on the transparentsubstrate, the phase adjusting film, the transmittance adjusting filmand the transparent substrate are etched, and therefore a mask patternwith any shape can be easily realized that has the semi-light-shieldingportion serving as a low transmittance phase shifter and the peripheralportion serving as a transmittance adjusting portion, and alight-transmitting portion with any shape can be easily realized thatserves as a high transmittance phase shifter. Furthermore, the step ofremoving the phase adjusting film in the peripheral portion formationregion and the step of removing the phase adjusting film in thelight-transmitting portion are performed separately, so that when thelight-transmitting portion formation region and the peripheral portionformation region are apart by a small distance, that is, when the phaseadjusting film having a small width is left between thelight-transmitting portion and the opening, a margin for photomaskprocessing can be increased.

A second method for producing a photomask of the present invention is amethod for producing a photomask including a semi-light-shieldingportion having a light-shielding property with respect to exposurelight, a light-transmitting portion surrounded by thesemi-light-shielding portion and having a light-transmitting propertywith respect to exposure light, and a peripheral portion surrounded bythe semi-light-shielding portion and positioned in a periphery of thelight-transmitting portion on a transparent substrate. Morespecifically, the method includes a first step of forming a phaseadjusting film that transmits the exposure light in a phase opposite tothat of the peripheral portion and a transmittance adjusting film havinga transmittance lower than that of the transparent substrate withrespect to the exposure light sequentially on the entire surface of thetransparent substrate, a second step of removing the phase adjustingfilm and the transmittance adjusting film on the peripheral portionformation region, and a third step of removing the transmittanceadjusting film in the light-transmitting portion formation region afterthe second step. The phase adjusting film and the transmittanceadjusting film formed on the transparent substrate in thesemi-light-shielding portion formation region constitute the phase shiftfilm that has a transmittance that allows the exposure light to betransmitted partially and transmits the exposure light in a phaseopposite to that of the peripheral portion.

According to the second method for producing a photomask, after thephase adjusting film and the transmittance adjusting film are formed onthe transparent substrate, the phase adjusting film and thetransmittance adjusting film in the peripheral portion formation regionare removed. Then, the transmittance adjusting film in thelight-transmitting portion formation region is removed. As a result, aphase shift film including the phase adjusting film and thetransmittance adjusting film, that is, a phase shift film that transmitsexposure light partially in an opposite phase is formed on thetransparent substrate in the semi-light-shielding portion formationregion, and a single layered structure of the phase adjusting film isformed on the transparent substrate in the light-transmitting portionformation region. Therefore, the peripheral portion that transmitsexposure light in a phase opposite to that of the light-transmittingportion is sandwiched by the light-transmitting portion serving as ahigh transmittance phase shifter and the semi-light-shielding portionserving as a low transmittance phase shifter that transmits exposurelight in the same phase as that of the light-transmitting portion.Consequently, the contrast in the light intensity distribution betweenthe light-transmitting portion and the peripheral portion can beenhanced by mutual interference between the light transmitted throughthe light-transmitting portion and the light transmitted through theperipheral portion. This contrast enhancement effect also can beobtained when a fine isolated resist-removed portion (i.e., a fineisolated space pattern corresponding to a light-transmitting portion) isformed with oblique incident exposure, for example, in the positiveresist process. That is to say, oblique incident exposure canminiaturize isolated space patterns and isolated line patterns or densepatterns at the same time. After the phase adjusting film and thetransmittance adjusting film are formed sequentially on the transparentsubstrate, the transmittance adjusting film and the phase adjusting filmare etched, and therefore a mask pattern with any shape can be easilyrealized that has the semi-light-shielding portion serving as a lowtransmittance phase shifter and the peripheral portion, and alight-transmitting portion with any shape can be easily realized thatserves as a high transmittance phase shifter.

In the first and second methods for producing a photomask, it ispreferable that the transmittance with respect to the exposure light ofthe phase shift film is 6% or more and 15% or less.

With this feature, the contrast enhancement effect can be obtainedreliably while preventing a reduction in thickness of the resist film inpattern formation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1G are diagrams illustrating the principle of the outlineenhancement method of the present invention.

FIGS. 2A to 2F are diagrams illustrating the dependence of theconventional image enhancement effect utilizing phase end on the shapeof a light source.

FIGS. 3A to 3F are diagrams illustrating the limit of the size of aphase shifter in the outline enhancement method of the presentinvention.

FIGS. 4A and 4B are diagrams illustrating the limit of the size of aphase shifter in the outline enhancement method of the presentinvention.

FIGS. 5A to 5F are diagrams illustrating the light intensitydistribution produced by exposure light incident from various lightsource positions in forming isolated patterns with an outlineenhancement mask of the present invention.

FIGS. 6A to 6F are diagrams illustrating the light intensitydistribution produced by exposure light incident from various lightsource positions in forming isolated patterns with a conventionalhalf-tone phase-shifting mask.

FIGS. 7A to 7F are diagrams illustrating the dependence of the contrastand the DOF on the transmittance of a semi-light shielding portion inthe outline enhancement mask of the present invention.

FIGS. 8A to 8F are diagrams illustrating variations of the layout of alight shielding mask patterns constituted by a semi-light shieldingportion and a phase shifter in the outline enhancement mask providedwith an opening corresponding to a contact pattern.

PIGS. 9A to 9F are diagrams illustrating variations of the layout of alight shielding mask patterns constituted by a low transmittance phaseshifter and an opening in the outline enhancement mask provided with ahigh transmittance phase shifter corresponding to a contact pattern.

FIG. 10A shows a view showing an example of a desired pattern to beformed with a photomask of a first embodiment of the present invention.FIG. 10B is a plan view of the photomask of the first embodiment of thepresent invention. FIG. 10C is a cross-sectional view taken along lineAA′ in FIG. 10B.

FIG. 11A is a cross-sectional view when the phase shift film is a singlelayered film in the photomask of the first embodiment of the presentinvention. FIG. 11B is a cross-sectional view when the phase shift filmis a layered film of a transmittance adjusting film and a phaseadjusting film in the photomask of the first embodiment of the presentinvention.

FIG. 12A is a view showing the shape of a regular exposure light source.FIG. 12B is a view showing the shape of an annular exposure lightsource. FIG. 12C is a view showing the shape of a quadrupole exposurelight source. FIG. 12D is a view showing the shape of anannular—quadrupole mixed type exposure light source.

FIGS. 13A to 13D are cross-sectional views showing the processes of amethod forming a pattern with the photomask of the first embodiment ofthe present invention.

FIGS. 14A to 14E are cross-sectional views showing the processes of amethod producing the photomask of the first embodiment of the presentinvention. FIG. 14F is a plan view corresponding to the cross-sectionalview of FIG. 14C, and FIG. 14G is a plan view corresponding to thecross-sectional view of FIG. 14E.

FIGS. 15A to 15E are cross-sectional views showing the processes of amethod producing the photomask of a first variation example of the firstembodiment of the present invention. FIG. 15F is a plan viewcorresponding to the cross-sectional view of FIG. 15C, and FIG. 15G is aplan view corresponding to the cross-sectional view of FIG. 15E.

FIGS. 16A to 16E are cross-sectional views showing the processes of amethod producing the photomask of a second variation example of thefirst embodiment of the present invention. FIG. 16F is a plan viewcorresponding to the cross-sectional view of FIG. 16C, and FIG. 16G is aplan view corresponding to the cross-sectional view of FIG. 16E.

FIGS. 17A and 17B are a plan view and a cross-sectional view of thephotomask of a third variation example of the first embodiment of thepresent invention, respectively. FIGS. 17C and 17D are a plan view and across-sectional view of the photomask of the third variation example ofthe first embodiment of the present invention in which the phaseadjusting film between the opening and the high transmittance phaseshifter has been removed, respectively.

FIG. 18A is a view showing an example of a desired pattern to be formedwith the photomask of a second embodiment of the present invention. FIG.18B is a plan view of the photomask of the second embodiment of thepresent invention. FIG. 18C is a cross-sectional view taken along lineAA′ of FIG. 18B.

FIGS. 19A to 19D are cross-sectional views showing the processes of amethod forming a pattern with the photomask of the second embodiment ofthe present invention.

FIGS. 20A to 20E are cross-sectional views showing the processes of amethod producing the photomask of the second embodiment of the presentinvention. FIG. 20F is a plan view corresponding to the cross-sectionalview of FIG. 20C, and FIG. 20G is a plan view corresponding to thecross-sectional view of FIG. 20E.

FIGS. 21A to 21E are cross-sectional views showing the processes of amethod producing the photomask of a first variation example of thesecond embodiment of the present invention. FIG. 21F is a plan viewcorresponding to the cross-sectional view of FIG. 21C, and FIG. 21G is aplan view corresponding to the cross-sectional view of FIG. 21E.

FIGS. 22A to 22E are cross-sectional views showing the processes of amethod producing the photomask of a second variation example of thesecond embodiment of the present invention. FIG. 22F is a plan viewcorresponding to the cross-sectional view of FIG. 22C, and FIG. 22G is aplan view corresponding to the cross-sectional view of FIG. 22E.

FIG. 23A is a view showing an example of a desired pattern to be formedwith the photomask of a third embodiment of the present invention. FIG.23B is a plan view of the photomask of the third embodiment of thepresent invention. FIG. 23C is a cross-sectional view taken along lineAA′ of FIG. 23B.

FIGS. 24A to 24D are cross-sectional views showing the processes of amethod forming patterns with the photomask of the third embodiment ofthe present invention.

FIGS. 25A to 25E are cross-sectional views showing the processes of amethod producing the photomask of the third embodiment of the presentinvention. FIG. 25F is a plan view corresponding to the cross-sectionalview of FIG. 25C, and FIG. 25G is a plan view corresponding to thecross-sectional view of FIG. 25E.

FIGS. 26A to 26C are diagrams illustrating the influence of a phasechange caused by use of a thin light-shielding film as the transmittanceadjusting film of the photomask of the third embodiment of the presentinvention on the formation of patterns.

FIGS. 27A to 27G are diagrams illustrating the image enhancementprinciple with a conventional half-tone phase-shifting mask.

DETAILED DESCRIPTION OF THE INVENTION

First, a method for improving the resolution with the photomask inventedby the inventors of the present application to realize the presentinvention, more specifically, an “outline enhancement method” to improvethe resolution of isolated space patterns will be described below.

Outline Enhancement Method

Hereinafter, the outline enhancement method will be described by takingformation of contact patterns by a positive resist process as anexample. The “outline enhancement method” is a principle that can beused for any patterns, regardless of its shape, as long as the patternsare small space patterns in a positive resist process. Furthermore, the“outline enhancement method” can be applied to a negative resist processtotally in the same manner, if the small space patterns (resist-removedpatterns) in the positive resist process are replaced by small patterns(resist patterns).

FIGS. 1A to 1G are diagrams illustrating the principle to enhance thecontract of transferred images of light in exposure for forming contactpatterns.

FIG. 1A is a plan view of a photomask in which an opening (i.e.,light-transmitting portion) corresponding to a contact pattern issurrounded by a semi-light-shielding portion having a transmittance of6% or more and 15% or less with respect to exposure light. FIG. 1B showsthe amplitude intensity corresponding to line AA′ of light transmittedthrough the photomask shown in FIG. 1A.

FIG. 1C is a plan view of a photomask in which a phase shifter isdisposed in a peripheral area of the opening shown in FIG. 1A, and acomplete light-shielding portion is disposed in the other area. FIG. 1Dshows the amplitude intensity corresponding to line AA′ of lighttransmitted through the photomask shown in FIG. 1C. The amplitudeintensity of light shown in FIG. 1D is that of the light transmittedthrough a phase shifter, and therefore this amplitude intensity has anopposite phase with respect to the amplitude intensity of light shown inFIG. 1B.

FIG. 1E is a plan view of a photomask in which an opening correspondingto a contact pattern and a phase shifter disposed in the peripheral areaof the opening are surrounded by a semi-light-shielding portion having atransmittance of 6% or more and 15% or less with respect to exposurelight. FIGS. 1F and 1G are the amplitude intensity and the lightintensity (a square of the amplitude intensity of light) correspondingto line AA′ of light transmitted through the photomask shown in FIG. 1E.The photomask shown in FIG. 1E is a photomask obtained by disposing aphase shifter in a peripheral area of the opening in the photomask shownin FIG. 1A. The photomask shown in FIG. 1E is an example of thephotomask of the present invention that can realize the outlineenhancement method (hereinafter, referred to as “outline enhancementmask”);

The photomask shown in FIG. 1A or 1E, the light transmitted through thesemi-light shielding portion and the light transmitted through theopening have the same phase (more specifically, a phase difference of(−30+360×n) degrees or more and (30+360×n) degrees or less, where n=aninteger). In the photomask shown in FIG. 1E, the light transmittedthrough the phase shifter and the light transmitted through the openinghave opposite phases (more specifically, a phase difference of(150+360×n) degrees or more and (210+360×n) degrees or less, where n=aninteger).

The principle based on which transferred image of light transmittedthrough the outline enhancement mask shown in FIG. 1E is as follows. Thestructure of the photomask shown in FIG. 1E is a structure in which thephotomasks shown in FIGS. 1A and 1C are overlapped each other.Therefore, as shown in FIGS. 1B, 1D, and 1F, the amplitude intensity oflight transmitted through the photomask shown in FIG. 1E has adistribution similar to that obtained by overlapping the amplitudeintensities of the lights transmitted through the photomasks shown inFIGS. 1A and 1C. As seen from FIG. 1F, in the photomask shown in FIG.1E, if the intensity of light transmitted through the phase shifterdisposed in the periphery of the opening can cancel a part of each ofthe lights transmitted through the opening and the semi-light shieldingportion. Therefore, in the photomask shown in FIG. 1E, if the intensityof the light transmitted through the phase shifter is adjusted such thatlight in the periphery of the opening is canceled, it is possible toform a light intensity distribution in which the light intensitycorresponding to the periphery of the opening is reduced to nearly 0, asshown in FIG. 1G.

In the photomask shown in FIG. 1E, the light transmitted through thephase shifter cancels the light in the periphery of the opening to ahigh degree, but cancels the light in the vicinity of the center of theopening to a low degree. As a result, there is another advantage thatthe slope of the profile of the light intensity distribution of thelight transmitted through the photomask shown in FIG. 1E in which thelight intensity changes from the center of the opening to the peripheryof the opening is increased, as shown in FIG. 1G. Therefore, the lightintensity distribution of the light transmitted through the photomaskshown in FIG. 1E has a sharp profile, so that images having a highcontrast can be formed.

Above described is the principle based on which optical images (imagesof light intensity) in the present invention are enhanced. In otherwords, a phase shifter is disposed along the outline of an opening in amask formed of a semi-light shielding portion having a lowtransmittance, so that it is possible to form a very dark portioncorresponding to the outline of the opening in a light intensity imageformed with the photomask shown in FIG. 1A. Thus, a light intensitydistribution in which the contrast between the light intensity in theopening and the light intensity in the periphery of the opening isenhanced can be formed. In this specification, a method by which imageenhancement is performed based on this principle is referred to as the“outline enhancement method”, and the photomask that realizes thisprinciple is referred to as an “outline enhancement mask”.

Hereinafter, the difference between the outline enhancement method,which is the basic principle of the present invention, and the principleof a conventional method using a half-tone phase-shifting mask will bedescribed. The most important point of the principle of the outlineenhancement mask is that a part of the light transmitted through each ofthe semi-light shielding portion and the opening is canceled by thelight transmitted through the phase shifter, so that a dark portion isformed in the light intensity distribution, that is, that the phaseshifter behaves in a manner similar to a non-transparent pattern (opaquepattern). Therefore, as shown in FIG. 1F, a dark portion is formed by achange in intensity on the same phase side in the amplitude intensity ofthe light transmitted through the outline enhancement mask. Only in thisstate, the contrast can be improved by oblique incident exposure, whichwill be described in detail later.

On the other hand, also in the light intensity distribution obtained byexposure with the conventional half-tone phase-shifting mask having anopening corresponding to a contact pattern, a very dark portion isformed in the periphery of the opening, as shown in FIG. 27G. However,when the amplitude intensity of the light shown in FIG. 27F obtained byexposure with the half-tone phase-shifting mask is compared with theamplitude intensity of the light shown in FIG. 1F obtained by exposurewith the outline enhancement mask, the following difference is clearlypresent. As shown in FIG. 27F, in the amplitude intensity distributionobtained by exposure with the half-tone phase-shifting mask, a phaseboundary in which a phase inversion occurs is present. As shown in FIG.27G, this phase boundary constitutes a dark portion of the lightintensity distribution due to the phase end and thus image enhancementis realized. However, in order to form a dark portion due to the phaseend to obtain an enhancement effect of the contract, a component oflight incident vertically to the photomask is required. On the otherhand, oblique incident exposure cannot provide a dark portion due to aphase end, even if the phase boundary is generated, and consequently thecontrast enhancement effect cannot be obtained. This is the reason whythe contrast enhancement effect cannot be obtained when oblique incidentexposure is performed with the half-tone phase-shifting mask. In otherwords, in order to obtain the contrast enhancement effect with thehalf-tone phase-shifting mask, it is necessary to perform exposure usinga small light source having a low coherence degree.

As described above, in forming contact patterns, although the lightintensity distribution with the half-tone phase-shifting mask is similarto that with the outline enhancement mask, the outline enhancementmethod can provide a higher contrast to a transferred image of light,which is necessary for forming small isolated space patterns, even withoblique incident exposure, because of the difference in the principlefor formation of a dark portion (the phase boundary is not generated inthe amplitude intensity distribution of the light transmitted throughthe outline enhancement mask (see FIG. 1F).

FIG. 2A is a plan view of a half-tone phase-shifting mask in which anopening corresponding to a contact pattern is surrounded by a phaseshifter. FIG. 2B shows calculation results of the light intensitydistribution corresponding to line AA′ when exposure is performed with asmall light source having a small coherence degree σ=0.4 with respect tothe half-tone phase-shifting mask shown in FIG. 2A. FIG. 2C showscalculation results of the light intensity distribution corresponding toline AA′ when exposure is performed with annular illumination, which isone type of oblique incident exposure, with respect to the half-tonephase-shifting mask shown in FIG. 2A. In this case, what is called ⅔annular illumination having an outer diameter σ of 0.75 and an innerdiameter σ of 0.5 is used as the annular illumination. For the exposureconditions, the light source wavelength λ is 193 nm (ArF light source)and the numerical aperture NA is 0.6. The contact size is 180 nm square,and the transmittance of the phase shifter is 6%. In the followingdescription, the light intensity is shown by a relative light intensitywhen taking the light intensity of exposure light as 1, unless otherwisespecified.

As shown in FIGS. 2B and 2C, when the half-tone phase-shifting mask isused, a dark portion due to a phase end is formed in the light intensitydistribution from exposure with a small light source and an image havinga high contrast can be formed. On the other hand, in the light intensitydistribution from oblique incident exposure, a dark portion due to aphase end is not formed, and therefore an image having a very poorcontrast is formed.

FIG. 2D is a plan view of an edge enhancement phase-shifting mask inwhich an opening corresponding to a contact pattern and a phase shifterpositioned in an area surrounding the opening are surrounded by achromium film serving as a complete light-shielding portion. FIG. 2Eshows calculation results of the light intensity distributioncorresponding to line AA′ when exposure is performed with a small lightsource having a small coherence degree σ=0.4 with respect to the edgeenhancement phase-shifting mask shown in FIG. 2D. FIG. 2F showscalculation results of the light intensity distribution corresponding toline AA′ when exposure is performed with annular illumination withrespect to the edge enhancement phase-shifting mask shown in FIG. 2E.Herein, similarly to the half-tone phase-shifting mask, the “edgeenhancement phase-shifting mask” is a mask that can realize imageenhancement by forming a dark portion due to a phase end between anopening and a phase shifter. The type of annular illumination, theexposure conditions and the transmission of the phase shifter are thesame as those in the case of the half-tone phase-shifting mask shown inFIGS. 2A to 2C. The contact size is 220 nm square, and the width of thephase shifter is 80 nm.

As shown in FIGS. 2E and 2F, when the edge enhancement phase-shiftingmask is used, similarly to the case of the half-tone phase-shiftingmask, a dark portion due to a phase end is formed in the light intensitydistribution from exposure with a small light source, and an imagehaving a high contrast can be formed. On the other hand, in the lightintensity distribution from oblique incident exposure, a dark portiondue to a phase end is not formed, and therefore an image having a verypoor contrast is formed.

Next, in the outline enhancement method, before showing in detail thatoblique incident exposure components can provide high contrast, the factthat the structure of the outline enhancement mask as shown in FIG. 1Ecannot provide the outline enhancement effect when the width of thephase shifter becomes too large will be described.

FIG. 3A is a plan view of an outline enhancement mask in which anopening corresponding to a contact pattern and a phase shifter having asmall width positioned in an area surrounding the opening are surroundedby a semi-light shielding portion having a transmittance of 6% or moreand 15% or less with respect to exposure light. FIG. 3B showscalculation results of the light intensity distribution corresponding toline AA′ when exposure is performed with a small light source having asmall coherence degree σ=0.4 with respect to the outline enhancementmask shown in FIG. 3A. FIG. 3C shows calculation results of the lightintensity distribution corresponding to line AA′ when exposure isperformed with annular illumination with respect to the outlineenhancement mask shown in FIG. 3A.

FIG. 3D is a plan view of an outline enhancement mask in which anopening corresponding to a contact pattern and a phase shifter having alarge width positioned in an area surrounding the opening are surroundedby a semi-light shielding portion having a transmittance of 6% or moreand 15% or less with respect to exposure light. FIG. 3E showscalculation results of the light intensity distribution corresponding toline AA′ when exposure is performed with a small light source having asmall coherence degree σ=0.4 with respect to the outline enhancementmask shown in FIG. 3D. FIG. 3F shows calculation results of the lightintensity distribution corresponding to line AA′ when exposure isperformed with annular illumination with respect to the outlineenhancement mask shown in FIG. 3D.

In this case, it is assumed that the width of the phase shifter in theoutline enhancement mask shown in FIG. 3D is set to be too large tosatisfy the principle of the outline enhancement method. Morespecifically, the sizes of the openings shown in FIGS. 3A and 3D areboth 220 nm square, and the width of the phase shifter shown in FIG. 3Ais 60 nm and the width of the phase shifter shown in FIG. 3D is 150 nm.The types of the annular illumination and the exposure conditions arethe same as those in the case of the half-tone phase-shifting mask shownin FIGS. 2A to 2C.

As shown in FIGS. 3B and 3C, when the outline enhancement mask shown inFIG. 3A that satisfies the principle of the outline enhancement methodis used, a dark portion due to a non-transparent function of the phaseshifter appears regardless of the type of the light source and thecontrast in the light intensity distribution is higher in the annularillumination.

On the other hand, when the outline enhancement mask shown in FIG. 3Dwith an excessively large phase shifter is used, the light transmittedthrough the phase shifter is too strong, so that an amplitude intensitydistribution having an opposite phase is formed. In this situation, thesame principle as in the case of the half-tone phase-shifting mask orthe edge enhancement phase-shifting mask acts. In other words, as shownin FIGS. 3E and 3F, a dark portion due to a phase end is formed in thelight intensity distribution obtained by exposure with a small lightsource and the contrast enhancement effect is provided, whereas no darkportion due to a phase end is formed in the light intensity distributionobtained by oblique incident exposure, so that an image having very poorcontrast is formed.

In other words, in order to realize the outline enhancement method, inthe mask structure, it is necessary that not only the phase shifter isdisposed in the periphery of the opening surrounded by the semi-lightshielding portion, but also that the light transmitted through the phaseshifter is limited. According to the mechanism of the principle, thelatter means that the light transmitted through the phase shifter has anintensity that at least can cancel the lights transmitted through thesemi-light shielding portion and the opening, and the intensitydistribution having an opposite phase with a predetermined size or moreis not formed in its amplitude intensity distribution.

In order to actually limit the light transmitted through the phaseshifter, a condition (more specifically the upper limit) can be imposedon the width of the phase shifter, depending on the transmittance of thephase shifter. Hereinafter, the condition will be described withreference to the results of observing conditions under which the lightfrom the periphery of the phase shifter is cancelled by the lighttransmitted through the phase shifter (see FIGS. 4A and 4B).

As shown in FIG. 4A, in exposure with a photomask (phase shifter mask)in which a phase shifter having a transmittance T and a line width L isprovided on a transparent substrate is used, the light intensitygenerated in a position corresponding to the center of the phase shifterin an exposed material is expressed as Ih (L, T). In exposure with aphotomask (light-shielding mask) in which the phase shifter of thephase-shifting mask is replaced by a complete light-shielding portion isused, the light intensity generated in a position corresponding to thecenter of the complete light-shielding portion in an exposed material isexpressed as Ic (L). In exposure with a photomask (light-transmittingmask) in which the phase shifter of the phase-shifting mask is replacedby an opening (light-transmitting portion) and the light-transmittingportion of the phase-shifting mask is replaced by a completelight-shielding portion is used, the light intensity generated in aposition corresponding to the center of the opening in an exposedmaterial is expressed as Io (L).

FIG. 4B is a graph showing the simulation results of the light intensityIh (L, T) when the transmittance T and the line width L of the phaseshifter are varied in exposure with the phase-shifting mask shown inFIG. 4A, represented by contour lines of the light intensity with thetransmittance T and the line width L in the vertical axis and thehorizontal axis, respectively. In this graph, a graph indicating therelationship of T=Ic (L)/Io (L) is superimposed. The simulationconditions are such that the wavelength of the exposure light λ=0.193 μm(ArF light source), the numerical aperture NA of the projection opticalsystem of the exposure apparatus =0.6, and the coherence degree σ of theexposure light source =0.8 (regular light source).

As shown in FIG. 4B, the condition under which the light intensity Ih(L, T) becomes smallest can be expressed by a relationship T=Ic (L)/Io(L). This physically represents a relationship in which T×Io (L)indicating the light intensity of the light transmitted through thephase shifter is in equilibrium with Ic (L) indicating the lightintensity of the light transmitted outside the phase shifter. Therefore,the width L of the phase shifter that provides an amplitude intensity ofan opposite phase in the amplitude intensity distribution because ofexcessive light transmitted through the phase shifter is a width L thatallows T×Io(L) to be larger than Ic (L).

It is empirically obtained from various simulation results that thewidth L that allows the light transmitted through the phase shifterhaving a transmittance of 1 to be in equilibrium with the lighttransmitted outside the phase shifter is about 0.3×λ (light sourcewavelength)/NA (numerical aperture) (about 100 nm in the case of FIG.4B), although this may depend on the type of the light source.Furthermore, as seen from FIG. 4B, in order to prevent too much lightfrom being transmitted through the phase shifter having a transmittanceof 6% (0.06) or more, the width L should be not more than twice thewidth of the phase shifter having a transmittance of 100% (1.0). That isto say, in order to prevent too much light from being transmittedthrough the phase shifter having a transmittance of 6% or more, theupper limit of the width L of the phase shifter should be not more than0.6×λ/NA.

If the above-described findings are applied to the outline enhancementmask, the upper limit of the width L of the phase shifter in the outlineenhancement mask can be considered to be a half of the upper limit inthe above findings because the light transmitted outside the phaseshifter in the outline enhancement mask to be taken into considerationis significantly only light on one side rather than both sides of thephase shifter. Therefore, the upper limit of the width L of the phaseshifter in the outline enhancement mask is not more than 0.3×λ/NA whenthe transmittance of the phase shifter is 6% or more. However, this isnot a sufficient condition, and the upper limit of the width L of thephase should be smaller than 0.3×λ/NA, depending on how high thetransmittance of the phase shifter is. That is to say, when thetransmittance of the phase shifter is as high as 100% or 50% or more,the width L of the phase shifter should be 0.2×λ/NA or less, preferably0.15×λ/NA or less. When forming fine hole patterns, in order to obtainthe effect of enhancing the profile of the light intensity distributionby interference between the light transmitted though the phase shifterand the light transmitted through the light-transmitting portioncorresponding to a hole pattern, it is preferable to arrange the phaseshifter in a region with a distance from the center of thelight-transmitting portion, that is, the hole of 0.5×λ/NA or less.Therefore, when the width L of the phase shifter is 0.3×λ/NA or less, itis preferable in forming hole patterns that the phase shiftersurrounding the light-transmitting portion is present in a region with adistance from the center of the light-transmitting portion correspondingto the hole pattern of 0.5×λ/NA or more and 0.8×λ/NA or less.

In this specification, unless otherwise specified, various mask sizessuch as the width of a phase shifter are shown by the sizes on anexposed material. The actual mask size can be obtained easily bymultiplying the sizes on an exposed material by the reduction ratio M ofa reduction projection optical system of an exposure apparatus.

Next, the image enhancement that can be realized with oblique incidentexposure in the outline enhancement method will be described in detail,based on a change in the contrast of the light intensity distributionwhen exposure is performed from various light source positions withrespect to the outline enhancement mask.

FIG. 5A is a plan view of the outline enhancement mask. In this case,the transmittance of the semi-light-shielding portion is 7.5%, and thetransmittance of the phase shifter and the opening is 100%. The size ofthe opening is 200 nm square, and the width of the phase shifter is 50nm.

FIG. 5C shows the results obtained by calculating the light intensitydistribution corresponding to line AA′ of FIG. 5A when exposure isperformed from a point light source in various positions normalized withthe numerical aperture NA with respect to the outline enhancement maskshown in FIG. 5A with optical simulations, reading the light intensityIo in a position corresponding to the center of the opening in thecalculation results (e.g., the light intensity distribution shown inFIG. 5B) and plotting the light intensity Io against each light sourceposition. The results shown in this plot are from the opticalcalculations that are performed assuming that the light sourcewavelength λ is 193 nm (ArF light source) and the numerical aperture NAis 0.6. In the following description, unless otherwise specified, in theoptical simulations, a calculation is performed under the conditionsthat the light source wavelength λ is 193 nm (ArF light source) and thenumerical aperture NA is 0.6.

As shown in FIG. 5C, the light intensity lo in the center of the openingis larger, as exposure is performed with a point light source in a lightposition on the outer side (a light source position more apart from theorigin in FIG. 5C). That is to say, the plot shows that as exposure isperformed with a light source having a larger oblique incidentcomponent, the contrast is larger. This will be described morespecifically with reference to the drawings. FIGS. 5D, 5E, and 5F aregraphs obtained by plotting the light intensity distributioncorresponding to line AA′ of FIG. 5A in sample points P1, P2 and P3 ofthe point light sources shown in FIG. 5C, respectively. As shown inFIGS. 5D, 5E, and 5F, as the position of the point light source is onthe outer side, in other words, as the light source is in the positionthat provides larger oblique incident light, an image of a highercontrast is formed.

Next, for comparison, a change in the contrast of the light intensitydistribution when exposure is performed from various light sourcepositions with respect to the half-tone phase-shifting mask will bedescribed. FIG. 6A is a plan view of the half-tone phase-shifting mask.In this case, the transmittance of the phase shifter is 6%, and thetransmittance of the opening is 100%. The size of the opening (size onan exposed wafer) is 180 nm square.

FIG. 6C shows the results obtained by calculating the light intensitydistribution corresponding to line AA′ of FIG. 6A when exposure isperformed from a point light source in various positions normalized withthe numerical aperture NA with respect to the half-tone phase-shiftingmask shown in FIG. 6A with optical simulations, reading the lightintensity Io in a position corresponding to the center of the opening inthe calculation results (e.g., the light intensity distribution shown inFIG. 6B) and plotting the light intensity Io against each light sourceposition.

As shown in FIG. 6C, the light intensity Io in the center of the openingis larger, as exposure is performed with a point light source in a lightposition on the inner side (a light source position closer to the originin FIG. 6C). That is to say, the plot shows that as exposure isperformed with a light source having a larger vertical incidentcomponent, the contrast is larger. This will be described morespecifically with reference to the drawings. FIGS. 6D, 6E, and 6F aregraphs obtained by plotting the light intensity distributioncorresponding to line AA′ of FIG. 6A in sample points P1, P2 and P3 ofthe point light sources shown in FIG. 6C, respectively. As shown inFIGS. 6D, 6E, and 6F, as the position of the point light source is onthe inner side, in other words, as the light source is in the positionthat provides larger vertical incident light, an image of a highercontrast is formed.

As seen from the comparison between the results shown in FIGS. 5A to 5Fand the results shown in FIGS. 6A to 6F, the outline enhancement methodmakes it possible to enhance the contrast of the light intensitydistribution obtained by oblique incident exposure in forming smallisolated space patterns such as contact patterns, which cannot berealized by the conventional methods.

The fact that the contrast is improved by the outline enhancement maskhas been described so far. Next, the dependence of the contrast and theDOF on the transmittance of the semi-light-shielding portion in theoutline enhancement mask will be described below. The followingdescription is based on the results obtained by simulations of variousmargins in pattern formation, using the outline enhancement mask shownin FIG. 7A. FIG. 7B shows the light intensity distribution formed whenexposure is performed with respect to the outline enhancement mask shownin FIG. 7A. In FIG. 7B, values regarding various margins defined whenforming a hole pattern with a width of 100 nm using the outlineenhancement mask shown in FIG. 7A are also shown. More specifically, thecritical intensity Ith is the light intensity that allows a resist filmto be exposed, and the margin is defined with respect to this value. Forexample, if Ip is the peak value of the light intensity distribution,Ip/Ith is proportional to the sensitivity with which a resist mask isexposed, and the higher value is more preferable. If Ib is thebackground intensity of light transmitted through thesemi-light-shielding portion, a higher Ith/Ib means that a reduction inthickness of the resist film hardly occurs at pattern formation, and thehigher value is more preferable. In general, it is preferable that avalue of Ith/Ib is at least 2. With the foregoing in mind, each marginwill be described.

FIG. 7C shows the calculation results regarding the dependence of theDOF on the transmittance of a semi-light-shielding portion in patternformation using the outline enhancement mask shown in FIG. 7A. Here, theDOF is defined as the width of the focus position in which a change inthe size of a finished pattern is within 10%. As shown in FIG. 7C, thehigher transmittance the semi-light-shielding portion has, the morepreferable it is for improvement of the DOF. FIG. 7D shows thecalculation results regarding the peak value Ip with respect to thetransmittance of the semi-light shielding portion in pattern formationusing the outline enhancement mask shown in FIG. 7A. As shown in FIG.7D, the higher transmittance the semi-light-shielding portion has, themore preferable it is for improvement of the peak value Ip, that is, thecontrast as well. From the above-described results, in the outlineenhancement mask, the higher transmittance the semi-light-shieldingportion has, the more preferable it is. More specifically, as shown inFIGS. 7C and 7D, the improvement rate of the exposure margin isincreased with an increase of the transmittance from 0% to about 6% andit can be appreciated that it is preferable to use asemi-light-shielding portion having a transmittance of about 6% or more.

FIG. 7E shows the calculation results regarding the Ith/Ib with respectto the transmittance of the semi-light shielding portion in patternformation using the outline enhancement mask shown in FIG. 7A. As shownin FIG. 7E, the higher transmittance the semi-light-shielding portionhas, the lower the value of Ith/Ib is. It is not preferable forimprovement of Ith/Ib that the transmittance is too high. Morespecifically, Ith/Ib is less than 2 when the transmittance of thesemi-light-shielding portion is about 15%. FIG. 7F shows the calculationresults regarding the Ip/Ith with respect to the transmittance of thesemi-light shielding portion in pattern formation using the outlineenhancement mask shown in FIG. 7A. As shown in FIG. 7F, the Ip/Ith has apeak at a transmittance of about 15% of the semi-light-shieldingportion.

As described above, in the outline enhancement mask, the DOF and thecontrast are improved more, as the transmittance of thesemi-light-shielding portion is higher, and this effect is moresignificant when the transmittance of the semi-light-shielding portionexceeds 6%. On the other hand, to prevent a reduction in thickness ofthe resist film during pattern formation or to optimize the resistsensitivity, it is preferable that the maximum of the transmittance ofthe semi-light-shielding portion is about 15%. Therefore, the optimalvalue of the transmittance of the semi-light-shielding portion in theoutline enhancement mask is 6% or more and 15% or less. That is to say,the semi-light-shielding portion transmits exposure light partially toan extent that the resist is not exposed. In other words, thesemi-light-shielding portion transmits a part of the total amount ofexposure light. Such a semi-light-shielding portion can be formed ofoxides such as ZrSiO, CrAlO, TaSiO, MoSiO or TiSiO.

FIGS. 8A to 8F are plan views showing variations of a light shieldingmask patterns constituted by a semi-light shielding portion and a phaseshifter in the outline enhancement mask provided with an openingcorresponding to a contact pattern.

An outline enhancement mask 1 a shown in FIG. 8A has the same structureof that of the outline enhancement mask shown in FIG. 1E. That is, theoutline enhancement mask 1 a is a photomask using a transparentsubstrate 2 a and includes a semi-light-shielding portion 3 a having atransmittance that allows a part of exposure light to be transmitted, anopening 4 a surrounded by the semi-light-shielding portion 3 a andcorresponding to an isolated contact pattern, and a ring-shaped phaseshifter 5 a positioned around the opening 4 a.

The outline enhancement mask 1 b shown in FIG. 8B is a photomask using atransparent substrate 2 b and includes a semi-light-shielding portion 3b having a transmittance that allows a part of exposure light to betransmitted, an opening 4 b surrounded by the semi-light-shieldingportion 3 b and corresponding to an isolated contact pattern, and aphase shifter 5 b constituted by four rectangular phase shifter portionsthat have a side having the same length of each side of the opening 4 band are in contact with the respective sides of the opening 4 b. Thisoutline enhancement mask 1 b has substantially the same characteristicsas those of the outline enhancement mask 1 a in isolated patternformation.

The outline enhancement mask 1 c shown in FIG. 8C is a photomask using atransparent substrate 2 c and includes a semi-light-shielding portion 3c having a transmittance that allows a part of exposure light to betransmitted, an opening 4 c surrounded by the semi-light-shieldingportion 3 c and corresponding to an isolated contact pattern, and aphase shifter 5 c constituted by four rectangular phase shifter portionsthat have a side having a length smaller than each side of the opening 4c and are in contact with the respective sides of the opening 4 c. Thecenter of each phase shifter portion of the phase shifter 5 c is alignedwith the center of the respective side of the opening 4 c. In thisoutline enhancement mask 1 c, the size of the resist pattern to beformed after exposure can be adjusted by changing the length of eachphase shifter portion of the phase shifter 5 c with the width (size) ofthe opening 4 c unchanged. For example, as the length of each phaseshifter portion of the phase shifter 5 c is smaller, the size of theresist pattern becomes larger. In this case, the lower limit withinwhich the length of each phase shifter portion of the phase shifter 5 ccan be changed without losing the function of outline enhancement islimited to about a half of the wavelength of the light source (exposurelight). On the other hand, since the pattern size is changed only to anextent of about a half of the change amount of the mask size, adjustingthe length of the phase shifter portion is an excellent approach toadjust the pattern size.

The outline enhancement mask 1 d shown in FIG. 8D is a photomask using atransparent substrate 2 d and includes a semi-light-shielding portion 3d having a transmittance that allows a part of exposure light to betransmitted, an opening 4 d surrounded by the semi-light-shieldingportion 3 d and corresponding to an isolated contact pattern, and aring-shaped phase shifter 5 d positioned apart from the boundary of thesemi-light-shielding portion 3 d and the opening 4 d by a predetermineddistance on the side of the semi-light-shielding portion 3 d. That is tosay, a ring-shaped semi-light-shielding portion 3 d is present betweenthe phase shifter 5 d and the opening 4 d.

The outline enhancement mask 1 e shown in FIG. 8E is a photomask using atransparent substrate 2 e and includes a semi-light-shielding portion 3e having a transmittance that allows a part of exposure light to betransmitted, an opening 4 e surrounded by the semi-light-shieldingportion 3 e and corresponding to an isolated contact pattern, and aphase shifter 5 e positioned apart from the boundary of thesemi-light-shielding portion 3 e and the opening 4 e by a predetermineddistance on the side of the semi-light-shielding portion 3 e. The phaseshifter 5 e is constituted by four phase shifter portions, each of whichis a rectangular shape having a length larger than each side of theopening 4 e and whose corner is in contact with the corners of theadjacent portions on the diagonal line of the opening 4 e. In this case,a ring-shaped semi-light-shielding portion 3 e is present between thephase shifter 5 e and the opening 4 e. In this outline enhancement mask1 e, the size of the resist pattern to be formed after exposure can beadjusted by changing only the width (size) of the opening 4 e with thesize and the arrangement of the phase shifter 5 e with unchanged. Forexample, as the width of the opening 4 e is increased, the size of theresist pattern is increased. According to this approach of adjusting thepattern size by changing only the width of the opening, MEEF (Mask ErrorEnhancement Factor: the ratio of the change amount of the pattern sizewith respect to the change amount of the mask size) is reduced to abouta half of that obtained by an approach of scaling both the opening andthe phase shifter at the same time to adjust the pattern size.

The outline enhancement mask 1 f shown in FIG. 8F is a photomask using atransparent substrate 2 f and includes a semi-light-shielding portion 3f having a transmittance that allows a part of exposure light to betransmitted, an opening 4 f surrounded by the semi-light-shieldingportion 3 f and corresponding to an isolated contact pattern, and aphase shifter 5 f positioned apart from the boundary of thesemi-light-shielding portion 3 f and the opening 4 f by a predetermineddistance on the side of the semi-light-shielding portion 3 f. The phaseshifter 5 f is constituted by four phase shifter portions, each of whichis a rectangular shape having the same length as that of each side ofthe opening 4 f and whose side is opposed to the corresponding side ofthe opening 4 f. In this case, the length of each phase shifter portionof the phase shifter 5 f may be larger or smaller than that of the sideof the opening 4 f. According to this outline enhancement mask 1 f, thesize of the resist pattern can be adjusted as in the case of the outlineenhancement mask 1 c shown in FIG. 8C.

In the outline enhancement masks shown in FIGS. 8D to 8F, in order toincrease the effect of reducing the MEEF, it is preferable that thewidth of the semi-light-shielding portion between the opening and thephase shifter is about ⅕ of λ/NA (λ is the wavelength of the exposurelight and NA is the numerical aperture). In order to obtain the effectof improving the DOF, it is preferable that the width of thesemi-light-shielding portion is a size that allows an interferenceeffect of light by the phase shifter to be provided, that is, about 1/10of λ/NA or less. In the outline enhancement masks shown in FIGS. 8A to8F, a square is used as the shape of the opening. However, a polygonsuch as an octagon or a circle, or other shapes can be used. The shapeof the phase shifter is not limited to a continuous ring shape or aplurality of rectangles. For example, the phase shifter can be formed byaligning a plurality of square phase shifter portions.

All the above description has been based on the positive resist processin which the portion corresponding to a resist-removed portion in theoutline enhancement mask is defined as the opening. However, if a phaseshifter having a sufficiently high transmittance can be used, in theoutline enhancement mask used for the above description, the portiondefined as the opening can be replaced by a phase shifter having a hightransmittance, the portion defined as the phase shifter can be replacedby an opening, and the portion defined as the semi-light-shieldingportion can be replaced by a phase shifter having a low transmittance(e.g., a phase shifter of a half-tone phase-shifting mask). In thiscase, the relationship of the relative phase difference between theelements is the same as in the above-described case, so that an outlineenhancement mask having the same effect can be realized. FIGS. 9A to 9Fare plan views showing variations of a light shielding mask patternsconstituted by a low transmittance phase shifter and an opening in theoutline enhancement mask provided with a high transmittance phaseshifter corresponding to a contact pattern. The masks shown in FIGS. 9Ato 9F have the structures in which the opening, the phase shifter andthe semi-light-shielding portion in the FIGS. 8A to 8F are replaced by ahigh transmittance phase shifter, an opening and a low transmittancephase shifter, respectively. In this case, it is preferable that thehigh transmittance phase shifter has a transmittance of at least 60%.That is to say, in the mask structure in which the high transmittancephase shifter is surrounded by the low transmittance phase shifter, inorder to have the low transmittance phase shifter correspond to anon-exposed portion of the resist film and the high transmittance phaseshifter correspond to an exposed portion of the resist film, thetransmittance of the high transmittance phase shifter should be about avalue at least three times, preferably ten times the transmittance ofthe low transmittance phase shifter. Therefore, it is preferable thatthe transmittance of the low transmittance phase shifter is 6 to 15%,whereas the transmittance of the high transmittance phase shifter is 60%or more. In the following embodiments, the outline enhancement masksshown in FIGS. 9A to 9F will be discussed.

FIRST EMBODIMENT

Hereinafter, a photomask according to a first embodiment of the presentinvention, a method for producing the photomask and a method for forminga pattern using the photomask will be described with reference to theaccompanying drawings. The photomask of the first embodiment is aphotomask of a reduction projection exposure system to realize theabove-described outline enhancement method.

FIG. 10A shows an example of a desired pattern to be formed with thephotomask of the first embodiment.

When the reduction ratio of a reduction projection optical system of anexposure apparatus is M, in a regular photomask, a pattern having a sizeM times the size of a desired pattern (in general, having a designedvalue on a wafer) is drawn on a substrate (transparent substrate) formedof a material having a high transmittance with respect to exposurelight, using a material, such as chromium film serving as a completelight-shielding portion with respect to the exposure light. However, inthis specification, for simplification, the present invention isdescribed, using the size on a wafer rather than using the size on themask, which is a size M times the size on a wafer, unless otherwisespecified. In this embodiment, when describing pattern formation, thedescription is based on the positive resist process, unless otherwisespecified. That is to say, the description is based on the assumptionthat an exposed portion of the resist film is removed. On the otherhand, when a negative resist process is assumed to be used, thedescription is totally the same as in the case of the positive resistprocess, except that the exposed portion of the resist film becomes aresist pattern. In this embodiment, the transmittance is expressed by aneffective transmittance when the transmittance of the transparentsubstrate is taken as 100%, unless otherwise specified.

FIG. 10B is a plan view of the photomask of the first embodiment, morespecifically, a photomask for forming the desired pattern shown in FIG.10A. As shown in FIG. 10B, high transmittance phase shifters(light-transmitting portions) are provided so as to correspond toresist-removed portions in the desired pattern. Furthermore, a lowtransmittance phase shifter (semi-light-shielding portion) having a lowtransmittance (6 to 15%) that does not allow the resist film to beexposed is used as the light-shielding mask pattern surrounding the hightransmittance phase shifter, instead of the complete light-shieldingportion that completely shields exposure light. Openings (peripheralportion) having a small width that is not provided with the lowtransmittance phase shifter are provided in the vicinity of the hightransmittance phase shifters. The high transmittance phase shifters andthe low transmittance phase shifter transmit exposure light in the samephase, whereas the openings transmits exposure light in an phaseopposite to those of the high transmittance phase shifters and the lowtransmittance phase shifter.

In the first embodiment, for example, as shown in FIG. 9B, the openingsare arranged in such a manner that the sides of the openings are incontact with the corresponding sides of the rectangular hightransmittance phase shifter in a region having a predetermine size orless from each side of the rectangular high transmittance phase shifter.

FIG. 10C is a cross-sectional view taken along line AA′ in FIG. 10B,that is a cross-sectional view of the photomask of the first embodiment.As shown in FIG. 10C, the photomask shown in FIG. 10B is realized in thefollowing manner. A phase shift film 11 having a low transmittance(about 6 to 15%) that does not allow the resist film to be exposed andhaving a phase difference of 180 degrees (more specifically (150+360×n)degrees or more and (210+360×n) degrees or less (where n is an integer))with respect to exposure light between this film and the transparentsubstrate 10 (opening) is formed on the portion of the transparentsubstrate 10 in the low transmittance phase shifter(semi-light-shielding portion) formation region. Thus, the lowtransmittance phase shifter is formed. The portion of the transparentsubstrate 10 in a light-transmitting portion formation region is dugdown by a thickness that causes a phase difference of 180 degrees (morespecifically (150+360×n) degrees or more and (210+360×n) degrees or less(where n is an integer)) with respect to exposure light between thisregion and the transparent substrate 10 (opening). Thus, the dug portion10 a of the transparent substrate 10 makes it possible to form thelight-transmitting portion serving as the high transmittance phaseshifter. Therefore, an outline enhancement mask having the followingstructure can be realized. The high transmittance phase shifter(light-transmitting portion) and the low transmittance phase shifter(semi-light-shielding portion) made of the phase shift film 11 sandwichthe peripheral portion not provided with the phase shift film 11 (thesurface of the transparent substrate 10 is exposed), that is, theopenings. As the phase shift film 11, a metal-containing oxide film suchas ZrSiO, CrAlO, TaSiO, MoSiO or TiSiO can be used. However, in order toobtain contrast enhancement by the outline enhancement method, it isnecessary to limit the width of the openings to a predetermined size orless.

In the above description, as shown in FIG. 11A, it is assumed that thephase shift film 11 serving as the low transmittance phase shifter is asingle layered film. In this case, the optical constant of the phaseshift film 11 is determined by the material, so that the thickness ofthe phase shift film 11 is determined by the amount of the phase shift.On the other hand, the transmittance depends on not only the opticalconstant, but also the film thickness, so that for the material of thephase shift film 11, a material having an appropriate optical constant,more specifically, a material that can achieve exactly the predeterminedtransmittance with the thickness that can transmit exposure light in aphase opposite to that of the transparent substrate 10 (opening) is notnecessarily present. Therefore, in the photomask according to the firstembodiment, as shown in FIG. 11B, it is preferable that the phase shiftfilm 11 has a two layered structure in which a transmittance adjustingfilm 11A having a low transmittance and a phase adjusting film 11Bhaving a high transmittance are laminated sequentially in order toachieve an arbitrary transmittance in the phase shift film 11. Morespecifically, the transmittance of the transmittance adjusting film 11Awith respect to exposure light is lower than that of the transparentsubstrate 10. The phase adjusting film 11B transmits exposure light in aphase opposite to that of the transparent substrate 10 (opening). As thetransmittance adjusting film 11A, a thin film (having a thickness of 30nm or less) made of a metal such as Zr, Cr, Ta, Mo or Ti or a thin film(having a thickness of 30 nm or less) made of a metal alloy such as aTa—Cr alloy, a Zr—Si alloy, a Mo—Si alloy or a Ti—Si alloy can be used.As the phase adjusting film 11B, an oxide film such as Sio₂ film can beused.

In this specification, a transmittance adjusting film refers to a filmthat has relatively a low transmittance per unit thickness with respectto exposure light and can set the transmittance with respect to exposurelight to a desired value by adjusting the thickness without affectingthe phase change with respect to the exposure light. A phase adjustingfilm refers to a film that has relatively a high transmittance per unitthickness with respect to exposure light and can set the phasedifference with respect to exposure light between this film and thetransparent substrate (opening) to a desired value by adjusting thethickness without affecting the transmittance change with respect to theexposure light.

Next, a method for forming a pattern using the photomask of the firstembodiment will be described. As described with reference to theprinciple of the outline enhancement method, when transferring a maskpattern in a reduced size with an exposure apparatus, it is preferableto use an oblique incident exposure light source in order to form animage having a high contrast with the outline enhancement mask. Herein,“oblique incident exposure” refers to light sources shown in FIGS. 12Bto 12D in which vertical incident components are removed, as opposed toa regular exposure light source as shown in FIG. 12A. Representativeoblique incident exposure light sources are an annular exposure lightsource shown in FIG. 12B and a quadrupole exposure light source shown inFIG. 12C. Although it depends slightly on a desired pattern, in general,quadrupole exposure light sources are more advantageous in enhancementof the contrast and enlargement of the DOF than annular exposure lightsources. However, quadrupole exposure light sources have such sideeffects that a pattern shape is distorted from the mask shape, so thatin such a case, it is preferable to use an annular-quadrupole mixed typeexposure light source as shown in FIG. 12D. The annular-quadrupole mixedtype exposure light source is characterized by having a feature of aquadrupole light source that the center of the light source and thelight sources on the XY axis are removed when assuming the XY coordinatewith the center of the light source (center of a regular exposure lightsource) as the origin, and having a feature of an annular light sourcethat a circle is used as the contour of the light source.

FIGS. 13A to 13D are cross-sectional views showing the processes of amethod forming patterns with the photomask of the first embodiment.

First, as shown in FIG. 13A, after a film 101 to be processed such as ametal film or an insulating film is formed on a substrate 100, as shownin FIG. 13B, a positive resist film 102 is formed on the film 101 to beprocessed.

Next, as shown in FIG. 13C, the photomask of the first embodimentincluding a low transmittance phase shifter made of the phase shift film11 and a light-transmitting portion functioning as a high transmittancephase shifter by the dug portion 10 a is irradiated with exposure light103 with an oblique incident exposure light source to expose the resistfilm 102 with transmitted light 104 transmitted through the photomask.In this case, as the mask pattern, the low transmittance phase shifter(semi-light-shielding portion) is used, so that the entire resist film102 is exposed with weak energy. However, as shown in FIG. 13C, only alatent image portion 102 a of the resist film 102 corresponding to thelight-transmitting portion (dug portion 10 a) in the photomask isirradiated with the exposure energy that is sufficient to dissolve theresist film 102 in a developing process.

Next, the latent image portion 102 a is removed by performingdevelopment with respect to the resist film 102, so that as shown inFIG. 13D, a resist pattern 105 is formed. In this case, in the exposureprocess shown in FIG. 13C, light around the light-transmitting portionis canceled, so that a portion corresponding to the opening (peripheralportion) in the resist film 102 is substantially not irradiated withexposure energy. Therefore, the contrast in the light intensitydistribution between the light transmitted through thelight-transmitting portion and the light transmitted through theperipheral portion, in other words, the contrast in the light intensitydistribution between the light with which the latent image portion 102 ais irradiated and the light with which the periphery of the latentportion 102 a is irradiated can be enhanced. Therefore, the energydistribution in the latent portion 102 a is changed sharply, so that aresist pattern 105 having a sharp shape can be formed.

Next, a method for producing a photomask of the first embodiment will bedescribed with reference to the drawings.

FIGS. 14A to 14E are cross-sectional views showing the processes of amethod producing the photomask of the first embodiment. FIG. 14F is aplan view corresponding to the cross-sectional view of FIG. 14C, andFIG. 14G is a plan view corresponding to the cross-sectional view ofFIG. 14E.

First, as shown in FIG. 14A, a phase shift film 11 having apredetermined transmittance (e.g., 6 to 15%) with respect to exposurelight is formed on a transparent substrate 10 made of a material havinglight-transmitting properties with respect to exposure light, such asquartz. As the phase shift film 11, a metal-containing oxide film suchas ZrSiO, CrAlO, TaSiO, MoSiO or TiSiO can be used. Furthermore, thephase shift film 11 generates a phase difference of (150+360×n) degreesor more and (210+360×n) degrees or less (where n=an integer) withrespect to exposure light between this film and the transparentsubstrate 10 (opening). In this embodiment, the phase shift film 11 mayhave a two layered structure of a transmittance adjusting film and aphase adjusting film, as described above.

Next, as shown in FIG. 14B, a first resist pattern 12 that covers thelow transmittance phase shifter (semi-light-shielding portion) formationregion is formed on the transparent substrate 10. That is, a firstresist pattern 12 having a removed portion in each of the hightransmittance phase shifter (light-transmitting portion) formationregion and the opening (peripheral portion) formation region is formedon the transparent substrate 10. Thereafter, the phase shift film 11 isetched with the first resist pattern 12 as a mask to pattern the phaseshift film 11. Then, the first resist pattern 12 is removed. Thus, asshown in FIGS. 14C and 14F, the portion corresponding to each of thehigh transmittance phase shifter formation region and the openingformation region in the phase shift film 11 is removed.

Next, as shown in FIG. 14D, a second resist pattern 13 that covers thelow transmittance phase shifter formation region and the openingformation region is formed on the transparent substrate 10. That is, asecond resist pattern 13 having a removed portion in the hightransmittance phase shifter formation region is formed. Thereafter, thetransparent substrate 10 is etched with the second resist pattern 13 asa mask. Then, the second resist pattern 13 is removed. Thus, as shown inFIGS. 14E and 14G, the dug portion 10 a that generates a phase inversionof 180 degrees (more specifically, (150+360×n) degrees or more and(210+360×n) degrees or less (where n is an integer)) is formed in theportion corresponding to the high transmittance phase shifter formationregion in the transparent substrate 10, and thus the photomask of thefirst embodiment is completed. That is to say, the photomask of thefirst embodiment having a plane structure of the outline enhancementmask can be easily formed by, as a mask blank, preparing a transparentsubstrate in which a phase shift film is deposited, that is, the samesubstrate as the conventional half-tone phase-shifting mask, and thenperforming etching with respect to the phase shift film and thetransparent substrate sequentially.

As described above, according to the first embodiment, the phase shiftfilm 11 that transmits exposure light at a low transmittance with aphase inversion is formed on the portion of the transparent substrate 10in the low transmittance phase shifter (semi-light-shielding portion)formation region. Furthermore, the portion of the transparent substrate10 in the light-transmitting portion formation region is dug down by thethickness that causes the exposure light to have a phase inversion sothat a light-transmitting portion is formed. Therefore, the opening thatis not provided with the phase shift film 11, that is, the peripheralportion that transmits exposure light in the phase opposite to that ofthe light-transmitting portion is sandwiched by the light-transmittingportion that serves as the high transmittance phase shifter by the dugportion 10 a and the low transmittance phase shifter that transmitsexposure light in the same phase as that of the light-transmittingportion and is made of the phase shift film 11. As a result, thecontrast in the light intensity distribution between thelight-transmitting portion and the peripheral portion can be enhanced bymutual interference between the light transmitted through the peripheralportion and the light transmitted through the light-transmittingportion. This contrast enhancement effect also can be obtained when afine isolated resist-removed portion (i.e., a fine isolated spacepattern) is formed with oblique incident exposure (off-axisillumination), for example, in the positive resist process. That is tosay, a combination of the photomask of this embodiment and obliqueincident exposure can miniaturize isolated space patterns and isolatedline patterns or dense patterns at the same time.

According to the first embodiment, after the phase shift film 11 isformed on the transparent substrate 10, the phase shift film 11 and thetransparent substrate 10 are etched selectively, and therefore a maskpattern with any shape can be easily realized that has the lowtransmittance phase shifter and the opening, and a light-transmittingportion with any shape can be easily realized that serves as the hightransmittance phase shifter.

According to the first embodiment, an opening with any shape can beformed by processing the phase shift film 11 constituting the lowtransmittance phase shifter, so that as the pattern layout of theoutline enhancement mask, not only the type shown in FIGS. 10B and 10C,that is, the type shown in FIG. 9B, but also all the types shown inFIGS. 9A to 9F, for example, can be realized.

In the first embodiment, it is preferable that the transmittance of thephase shift film 11, that is, the low transmittance phase shifter is 6%or more and 15% or less. Thus, the contrast enhancement effect can beobtained reliably while preventing a reduction in thickness of theresist film in pattern formation.

In the first embodiment, it is preferable that the phase shift film 11has a two layered structure in which the transmittance adjusting film11A having a low transmittance and the phase adjusting film 11B having ahigh transmittance are laminated sequentially. Thus, a combination of adesired phase change and a desired transmittance can be selectedarbitrarily for the phase shift film 11. A combination of the materialof the transmittance adjusting film 11A and the material of the phaseadjusting film 11B makes it possible to improve the selection ratio atetching for processing the phase shift film 11.

In the first embodiment, the description is based on the use of thepositive resist process, but the negative resist process can be used,instead of the positive resist process. In this case, in either one ofthe processes, as the exposure light source, the i line (wavelength 365nm), KrF excimer laser light (wavelength 248 nm), ArF excimer laserlight (wavelength 193 nm), or F₂ excimer laser light (wavelength 157 nm)can be used, for example.

First Variation of the First Embodiment

Hereinafter, a photomask of a first variation of the first embodimentand a method for producing the photomask will be described withreference to the accompanying drawings.

The first variation of the first embodiment is different from the firstembodiment in the following aspects. In the first embodiment, theoutline enhancement mask having a layout in which the high transmittancephase shifter and the opening are adjacent as shown, for example, inFIGS. 9A to 9C is described. In the first variation of the firstembodiment, the outline enhancement mask having a layout in which thehigh transmittance phase shifter (light-transmitting portion) and theopening (peripheral portion) are apart as shown, for example, in FIGS.9D to 9F is described.

FIGS. 15A to 15E are cross-sectional views showing the processes of amethod producing a photomask of the first variation example of the firstembodiment. FIG. 15F is a plan view corresponding to the cross-sectionalview of FIG. 15C, and FIG. 15G is a plan view corresponding to thecross-sectional view of FIG. 15E.

First, as shown in FIG. 15A, a phase shift film 11 having apredetermined transmittance (e.g., 6 to 15%) with respect to exposurelight is formed on a transparent substrate 10 made of a material havinglight-transmitting properties with respect to exposure light, such asquartz. The phase shift film 11 generates a phase difference of(150+360×n) degrees or more and (210+360×n) degrees or less (where n=aninteger) with respect to exposure light between this film and thetransparent substrate 10 (opening). In this example, the phase shiftfilm 11 may have a two layered structure of a transmittance adjustingfilm and a phase adjusting film (see the first embodiment).

Next, as shown in FIG. 15B, a first resist pattern 12 that covers a lowtransmittance phase shifter (semi-light-shielding portion) formationregion is formed on the transparent substrate 10. That is, a firstresist pattern 12 having a removed portion in each of a hightransmittance phase shifter (light-transmitting portion) formationregion and an opening (peripheral portion) formation region is formed.In this example, the opening formation region and the high transmittancephase shifter formation region are apart. In other words, the firstresist pattern 12 is interposed between the opening formation region andthe high transmittance phase shifter formation region. Thereafter, thephase shift film 11 is etched with the first resist pattern 12 as a maskto pattern the phase shift film 11. Then, the first resist pattern 12 isremoved. Thus, as shown in FIGS. 15C and 15F, the portion correspondingto each of the high transmittance phase shifter formation region and theopening formation region in the phase shift film 11 is removed.

Next, as shown in FIG. 15D, a second resist pattern 13 that covers thelow transmittance phase shifter formation region including the openingformation region and that has a removed portion in the hightransmittance phase shifter formation region is formed on thetransparent substrate 10. Thereafter, the transparent substrate 10 isetched with the second resist pattern 13 and the patterned phase shiftfilm 11 as masks. Then, the second resist pattern 13 is removed. Thus,as shown in FIGS. 15E and 15G, a dug portion 10 a that generates a phaseinversion of 180 degrees (more specifically, (150+360×n) degrees or moreand (210+360×n) degrees or less (where n is an integer)) is formed inthe portion corresponding to the high transmittance phase shifterformation region in the transparent substrate 10, and thus the photomaskof the first variation of the first embodiment is completed.

According to the first variation of the first embodiment, the followingadvantages can be obtained, in addition to those of the firstembodiment. Since the patterned phase shift film 11 is used as a maskfor etching the transparent substrate 10 in a self-alignment manner,photomask process can be performed precisely.

Second Variation of the First Embodiment

Hereinafter, a photomask of a second variation of the first embodimentand a method for producing the photomask will be described withreference to the accompanying drawings.

The second variation of the first embodiment is different from the firstembodiment in the following aspects. In the first embodiment, theoutline enhancement mask having a layout in which the high transmittancephase shifter (light-transmitting portion) and the opening (peripheralportion) are adjacent as shown, for example, in FIGS. 9A to 9C isdescribed. In the second variation of the first embodiment as well asthe first variation of the first embodiment, the outline enhancementmask having a layout in which the high transmittance phase shifter andthe opening are apart as shown, for example, in FIGS. 9D to 9F isdescribed.

FIGS. 16A to 16E are cross-sectional views showing the processes of amethod producing a photomask of the second variation example of thefirst embodiment. FIG. 16F is a plan view corresponding to thecross-sectional view of FIG. 16C, and FIG. 16G is a plan viewcorresponding to the cross-sectional view of FIG. 16E.

First, as shown in FIG. 16A, a phase shift film 11 having apredetermined transmittance (e.g., 6 to 15%) with respect to exposurelight is formed on a transparent substrate 10 made of a material havinglight-transmitting properties with respect to exposure light, such asquartz. The phase shift film 11 generates a phase difference of(150+360×n) degrees or more and (210+360×n) degrees or less (where n=aninteger) with respect to exposure light between this film and thetransparent substrate 10 (opening). In this example, the phase shiftfilm 11 may have a two layered structure of a transmittance adjustingfilm and a phase adjusting film (see the first embodiment).

Next, as shown in FIG. 16B, a first resist pattern 12 that covers a lowtransmittance phase shifter (semi-light-shielding portion) formationregion and a high transmittance phase shifter (light-transmittingportion) formation region is formed on the transparent substrate 10.That is, a first resist pattern 12 having a removed portion in anopening (peripheral portion) formation region is formed on thetransparent substrate 10. Thereafter, the phase shift film 11 is etchedwith the first resist pattern 12 as a mask to pattern the phase shiftfilm 11. Then, the first resist pattern 12 is removed. Thus, as shown inFIGS. 16C and 16F, the portion corresponding to the opening formationregion in the phase shift film 11 is removed.

Next, as shown in FIG. 16D, a second resist pattern 13 that covers thelow transmittance phase shifter formation region and the openingformation region is formed on the transparent substrate 10. That is tosay, a second resist pattern 13 that has a removed portion in the hightransmittance phase shifter formation region is formed on thetransparent substrate 10. Thereafter, the phase shift film 11 and thetransparent substrate 10 are etched sequentially with the second resistpattern 13 as a mask. Then, the second resist pattern 13 is removed.Thus, as shown in FIGS. 16E and 16G, the portion corresponding to thehigh transmittance phase shifter formation region in the phase shiftfilm 11 is removed. Furthermore, a dug portion 10 a that generates aphase inversion of 180 degrees (more specifically, (150+360×n) degreesor more and (210+360×n) degrees or less (where n is an integer)) isformed in the portion corresponding to the high transmittance phaseshifter formation region in the transparent substrate 10, and thus thephotomask of the second variation of the first embodiment is completed.

According to the second variation of the first embodiment, the followingadvantages can be obtained, in addition to those of the firstembodiment. In this variation, the process of removing the portioncorresponding to the opening formation region in the phase shift film 11(see FIG. 16C) and the process of removing the portion corresponding tothe high transmittance phase shifter formation region in the phase shiftfilm 11 (see FIG. 16E) are performed separately. Therefore, if theopening is apart from the high transmittance phase shifter with a smalldistance, in other words, if the phase shift film 11 having a smallwidth is left between the opening and the high transmittance phaseshifter, the margin for photomask process becomes large.

In the second variation of the first embodiment, before performing theprocess of removing the portion corresponding to the opening formationregion in the phase shift film 11, the process of removing the portioncorresponding to the high transmittance phase shifter formation regionin the phase shift film 11 (including the process of forming a dugportion 10 a in the transparent substrate 10).may be performed.

Third Variation of the First Embodiment

Hereinafter, a photomask of a third variation of the first embodimentand a method for producing the photomask will be described withreference to the accompanying drawings.

The third variation of the first embodiment is different from the firstembodiment in the following aspects. In the first embodiment, theoutline enhancement mask having a layout in which the high transmittancephase shifter (light-transmitting portion) and the opening (peripheralportion) are adjacent as shown, for example, in FIGS. 9A to 9C isdescribed. In the third variation of the first embodiment, the outlineenhancement mask having a layout in which the high transmittance phaseshifter and the opening are apart as shown, for example, in FIGS. 9D to9F is described. Moreover, in the first embodiment, the phase shift film11 that will serve as the low transmittance phase shifter(semi-light-shielding portion) is assumed to be a single layered film,as shown in FIG. 11A. However, in the third variation of the firstembodiment, the phase shift film 11, for example, as shown in FIG. 11Bis assumed to have a two layered structure in which a transmittanceadjusting film 11A having a low transmittance and a phase adjusting film11B having a high transmittance are laminated sequentially.

FIGS. 17A and 17B are a plan view and a cross-sectional view of thephotomask of the third variation example of the first embodiment,respectively. As shown in FIGS. 17A and 17B, the light-transmittingportion serving as a high transmittance phase shifter due to the dugportion 10 a of the transparent substrate 10 and the opening that is notprovided with the phase shift film 11, that is, the peripheral portionare apart. The phase shift film 11 serving as a low transmittance phaseshifter includes a transmittance adjusting film 11A having a lowtransmittance, which is a lower layer, and a phase adjusting film 11Bhaving a high transmittance, which is an upper layer. In this example,the transmittance adjusting film 11A is formed of a single layered thinfilm that generates a phase difference of (−30+360×n) degrees or moreand (30+360×n) degrees or less (where n=an integer) with respect toexposure light between this film and the transparent substrate 10(opening). That is to say, the transmittance adjusting film 11Agenerates only a slight phase change in the transmitted light. As thetransmittance adjusting film 11A, a thin film (having a thickness of 30nm or less) made of a metal such as Zr, Cr, Ta, Mo or Ti or a thin film(having a thickness of 30 nm or less) made of a metal alloy such as aTa—Cr alloy, a Zr—Si alloy, a Mo—Si alloy or a Ti—Si alloy can be used.As the phase adjusting film 11B, an oxide film such as SiO₂ film can beused.

In the first embodiment including this variation, the dug portion 10 amakes it possible to form a high transmittance phase shifter having asufficiently high transmittance (e.g., 90 to 100%). However, due tolight scattering on the etched surface of the transparent substrate 10or the like, the effective transmittance of the high transmittance phaseshifter is slightly lower than that of the opening (i.e., transparentsubstrate 10). Consequently, the transmittance of the opening is thehighest, so that requirements for miniaturization of the opening becomestrict.

The following advantages can be obtained by removing the phase adjustingfilm 11B from the portion formed between the opening and the hightransmittance phase shifter (dug portion 10 a) in the phase shift film11, in other words, by having only the transmittance adjusting film 11Aleft between the peripheral portion and the light-transmitting portion,as shown in the plan view of FIG. 17C and the cross-sectional view ofFIG. 17D.

First, when the thickness of the transmittance adjusting film 11A issufficiently small, the light transmitted through the opening hassubstantially the same phase as that of the light transmitted throughthe removed portion of the phase adjusting film 11B (i.e., the portionin which only the transmittance adjusting film 11A is formed in thetransparent substrate 10). In this state, the region obtained bycombining the opening and the removed portion of the phase adjustingfilm 11B is substantially equivalent to the region having atransmittance averaged in proportion to the area of each element. Inthis case, the average of the transmittance of the opening and thetransmittance of the removed portion of the phase adjusting film 11B issmaller than the transmittance of the opening, so that the structureshown in FIG. 17D is equivalent to the structure shown in FIG. 17C (aquasi-opening having an effective transmittance lower than that of theopening is formed in the vicinity of a high transmittance phaseshifter). That is to say, the transmittance (effective transmittance) ofthe opening including the removed portion of the phase adjusting film11B is smaller than 1, so that a margin for size control of the openingcan be increased.

Furthermore, when the transmittance adjusting film 11A is made of asingle layered thin film, compared with the case where a transmittanceadjusting film having a multilayered structure is used, the peeling ofthe transmittance adjusting film 11A is prevented when the transmittanceadjusting film 11A having a small width is formed between the openingand the high transmittance phase shifter.

SECOND EMBODIMENT

Hereinafter, a photomask according to a second embodiment of the presentinvention, a method for producing the photomask and a method for forminga pattern using the photomask will be described with reference to theaccompanying drawings. The photomask of the second embodiment is aphotomask of a reduction projection exposure system to realize theabove-described outline enhancement method.

FIG. 18A shows an example of a desired pattern to be formed with thephotomask of the second embodiment.

In this embodiment, when describing pattern formation, the descriptionis based on the positive resist process, unless otherwise specified.That is to say, the description is based on the assumption that anexposed portion of the resist film is removed. On the other hand, when anegative resist process is assumed to be used, the description istotally the same as in the case of the positive resist process, exceptthat the exposed portion of the resist film becomes a resist pattern. Inthis embodiment, the transmittance is expressed by an effectivetransmittance when the transmittance of the transparent substrate istaken as 100%, unless otherwise specified.

FIG. 18B is a plan view of the photomask of the second embodiment, morespecifically, a photomask for forming the desired pattern shown in FIG.18A. As shown in FIG. 18B, high transmittance phase shifters(light-transmitting portions) are provided so as to correspond toresist-removed portions in the desired pattern. Furthermore, a lowtransmittance phase shifter (semi-light-shielding portion) having a lowtransmittance (6 to 15%) that does not allow the resist film to beexposed is used as the light-shielding mask pattern surrounding the hightransmittance phase shifter, instead of the complete light-shieldingportion that completely shields exposure light. Openings (peripheralportion) having a small width that is not provided with the lowtransmittance phase shifter are provided in the vicinity of the hightransmittance phase shifters. In the second embodiment, a transmittanceadjusting film having a lower transmittance with respect to exposurelight than that of the transparent substrate is formed in the opening,and thus the transmittance of the opening is adjusted to a smaller valuethan that of the transmittance of the transparent substrate.Hereinafter, this opening is referred to as a “transmittance adjustingportion”. The high transmittance phase shifter and the low transmittancephase shifter transmit exposure light in the same phase, whereas thetransmittance adjusting portion transmits exposure light in a phaseopposite to the high transmittance phase shifter and the lowtransmittance phase shifter.

In the second embodiment, the transmittance adjusting portions(openings) are arranged in such a manner that the sides of thetransmittance adjusting portions are in contact with the correspondingsides of the rectangular high transmittance phase shifter in a regionhaving a predetermine size or less from each side of the rectangularhigh transmittance phase shifter, for example, as shown in FIG. 9B.

FIG. 18C is a cross-sectional view taken along line AA′ in FIG. 18B,that is a cross-sectional view of the photomask of the secondembodiment. As shown in FIG. 18C, the photomask shown in FIG. 18B isrealized in the following manner. A semi-light-shielding film(transmittance adjusting film) 21 that has a lower transmittance withrespect to exposure light than that of the transparent substrate 20 anda phase adjusting film 22 are formed sequentially on the portion of thetransparent substrate 20 in a region other than the light-transmittingportion formation region. The phase adjusting film 22 generates a phasedifference of 180 degrees (more specifically (150+360×n) degrees or moreand (210+360×n) degrees or less (where n is an integer)) with respect toexposure light between this portion and a multilayered structure of thetransparent substrate 20 and the transmittance adjusting film 21 (i.e.,transmittance adjusting portion (peripheral portion)). Thus, a phaseshift film having a multilayered structure of the transmittanceadjusting film 21 and the phase adjusting film 22 and having a lowtransmittance (about 6% to 15%) that does not allow the resist film tobe exposed is formed, and thus a semi-light-shielding portion serving asa low transmittance phase shifter is formed. The phase adjusting film 22is not formed in the transmittance adjusting portion. It is preferablethat the transmittance adjusting film 21 is a thin film, but may be athick film having an arbitrary thickness. Furthermore, the portion ofthe transparent substrate 20 in a light-transmitting portion formationregion is dug down by a thickness that causes a phase difference of 180degrees (more specifically (150+360×n) degrees or more and (210+360×n)degrees or less (where n is an integer)) with respect to exposure lightbetween this region and the multilayered structure of the transparentsubstrate 20 and the transmittance adjusting film 21 (i.e.,transmittance adjusting portion). Thus, the dug portion 20 a of thetransparent 20 makes it possible to form the light-transmitting portionserving as a high transmittance phase shifter. Therefore, the hightransmittance phase shifter (light-transmitting portion) and the lowtransmittance phase shifter (semi-light-shielding portion) made of themultilayered structure of the transmittance adjusting film 21 and thephase adjusting film 22 (the phase shift film) sandwich thetransmittance adjusting portion not provided with the phase adjustingfilm 22 (i.e., having a single layered structure of the transmittanceadjusting film 21), and thus an outline enhancement mask is realized. Asthe transmittance adjusting film 21, a thin film (having a thickness of30 nm or less) made of a metal such as Zr, Cr, Ta, Mo or Ti or a thinfilm (having a thickness of 30 nm or less) made of a metal alloy such asa Ta—Cr alloy, a Zr—Si alloy, a Mo—Si alloy or a Ti—Si alloy can beused. As the phase adjusting film 22, an oxide film such as Sio₂ filmcan be used. However, in order to enhance contrast by the outlineenhancement method, it is necessary to limit the width of thetransmittance adjusting portion to a predetermined size or less.

Next, a method for forming a pattern using the photomask of the secondembodiment will be described.

FIGS. 19A to 19D are cross-sectional views showing the processes of amethod forming patterns with the photomask of the second embodiment.

First, as shown in FIG. 19A, after a film 201 to be processed such as ametal film or an insulating film is formed on a substrate 200, as shownin FIG. 19B, a positive resist film 202 is formed on the film to beprocessed 201.

Next, as shown in FIG. 19C, the photomask of the second embodimentincluding the low transmittance phase shifter made of the multilayeredstructure (phase shift film) of the transmittance adjusting film 21 andthe phase adjusting film 22, a transmittance adjusting portion having asingle layered structure of the transmittance adjusting film 21, and alight-transmitting portion functioning as a high transmittance phaseshifter by the dug portion 20 a is irradiated with exposure light 203with an oblique incident exposure light source to expose the resist film202 with transmitted light 204 transmitted through the photomask. Inthis case, as the mask pattern, the low transmittance phase shifter(semi-light-shielding portion) is used, so that the entire resist film202 is exposed with weak energy. However, as shown in FIG. 19C, only alatent image portion 202 a of the resist film 202 corresponding to thelight-transmitting portion (dug portion 20 a) in the photomask isirradiated with exposure energy that is sufficient to dissolve theresist film 202 in a developing process.

Next, the latent image portion 202 a is removed by performingdevelopment with respect to the resist film 202, so that as shown inFIG. 19D, a resist pattern 205 is formed. In this case, in the exposureprocess shown in FIG. 19C, light around the light-transmitting portionis canceled, so that a-portion corresponding to the periphery of thelight-transmitting portion (transmittance adjusting portion) in theresist film 202 is substantially not irradiated with exposure energy.Therefore, the contrast in the light intensity distribution between thelight transmitted through the light-transmitting portion and the lighttransmitted through the transmittance adjusting portion, in other words,the contrast in the light intensity distribution between the light withwhich the latent image portion 202 a is irradiated and the light withwhich the periphery of the latent portion 202 a is irradiated can beenhanced. Therefore, the energy distribution in the latent portion 202 ais changed sharply, so that a resist pattern 205 having a sharp shapecan be formed.

Next, a method for producing a photomask of the second embodiment willbe described with reference to the drawings.

FIGS. 20A to 20E are cross-sectional views showing the processes of amethod producing the photomask of the second embodiment. FIG. 20F is aplan view corresponding to the cross-sectional view of FIG. 20C, andFIG. 20G is a plan view corresponding to the cross-sectional view ofFIG. 20E.

First, as shown in FIG. 20A, a transmittance adjusting film 21 having atransmittance lower than that of the transparent substrate 20 withrespect to exposure light and a phase adjusting film 22 are formedsequentially on a transparent substrate 20 made of a material havinglight-transmitting properties with respect to exposure light, such asquartz. The phase adjusting film 22 generates a phase difference of(150+360×n) degrees or more and (210+360×n) degrees or less (where n=aninteger) with respect to exposure light between this film and themultilayered structure of the transparent substrate 20 and thetransmittance adjusting film 21. The phase shift film having amultilayered structure including the transmittance adjusting film 21 andthe phase adjusting film 22 constitutes a semi-light-shielding portionhaving a predetermined transmittance (e.g., 6 to 15%) with respect toexposure light.

Next, as shown in FIG. 20B, a first resist pattern 23 that covers thelow transmittance phase shifter (semi-light-shielding portion) formationregion is formed on the transparent substrate 20. That is, a firstresist pattern 23 having a removed portion in each of the hightransmittance phase shifter (light-transmitting portion) formationregion and the transmittance adjusting portion (peripheral portion)formation region is formed on the transparent substrate 20. Thereafter,the phase adjusting film 22 is etched with the first resist pattern 23as a mask to pattern the phase adjusting film 22. Then, the first resistpattern 23 is removed. Thus, as shown in FIGS. 20C and 20F, the portioncorresponding to each of the high transmittance phase shifter formationregion and the opening formation region in the phase adjusting film 22is removed.

Next, as shown in FIG. 20D, a second resist pattern 24 that covers thelow transmittance phase shifter formation region and the transmittanceadjusting portion formation region is formed on the transparentsubstrate 20. That is, a second resist pattern 24 having a removedportion in the high transmittance phase shifter formation region isformed. Thereafter, the transmittance adjusting film 21 and thetransparent substrate 20 are etched sequentially with the second resistpattern 24 as a mask. Then, the second resist pattern 24 is removed.Thus, as shown in FIGS. 20E and 20G, the portion corresponding to thehigh transmittance phase shifter formation region in the transmittanceadjusting film 21 is removed. Furthermore, the dug portion 20 a thatgenerates a phase inversion of 180 degrees (more specifically,(150+360×n) degrees or more and (210+360×n) degrees or less (where n isan integer)) between this portion and the multilayered structure of thetransparent substrate 20 and the transmittance adjusting film 21 (i.e.,the transmittance adjusting portion) is formed in the portioncorresponding to the high transmittance phase shifter formation regionin the transparent substrate 20, and thus the photomask of the secondembodiment is completed. That is to say, the photomask of the secondembodiment having a plane structure of the outline enhancement mask canbe easily formed by, as a mask blank, preparing a transparent substratein which a phase shift film including a transmittance adjusting film anda phase adjusting film is deposited, and then performing etching withrespect to the phase adjusting film, the transmittance adjusting filmand the transparent substrate sequentially.

As described above, according to the second embodiment, the phase shiftfilm (a multilayered structure of the transmittance adjusting film 21and the phase adjusting film 22) that transmits exposure light at a lowtransmittance with a phase inversion is formed on the portion of thetransparent substrate 20 in the low transmittance phase shifter(semi-light-shielding portion) formation region. Furthermore,.theportion of the transparent substrate 20 in the light-transmittingportion formation region is dug down by a thickness that causes theexposure light to have a phase inversion so that a light-transmittingportion is formed. Furthermore, a single layered structure of thetransmittance adjusting film 21 is formed on the portion of thetransparent substrate 20 in the transmittance adjusting film formationregion. Therefore, the transmittance adjusting portion having a singlelayered structure of the transmittance adjusting film 21, that is, theperipheral portion that transmits exposure light in the phase oppositeto that of the light-transmitting portion is sandwiched by thelight-transmitting portion that serves as the high transmittance phaseshifter by the dug portion 20 a and the low transmittance phase shifterthat transmits exposure light in the same phase as that of thelight-transmitting portion and is made of the phase shift film. As aresult, the contrast in the light intensity distribution between thelight-transmitting portion and the peripheral portion can be enhanced bymutual interference between the light transmitted through the peripheralportion and the light transmitted through the light-transmittingportion. This contrast enhancement effect also can be obtained when afine isolated resist-removed portion (i.e., a fine isolated spacepattern) is formed with oblique incident exposure, for example, in thepositive resist process. That is to say, a combination of the photomaskof this embodiment and oblique incident exposure can miniaturizeisolated space patterns and isolated line patterns or dense patterns atthe same time.

According to the second embodiment, the phase shift film serving as alow transmittance phase shifter has a multilayered structure of thetransmittance adjusting film 21 having a low transmittance and the phaseadjusting film 22 having a high transmittance. Therefore, a combinationof a desired phase change and a desired transmittance can be selectedarbitrarily for the phase shift film. A combination of the material ofthe transmittance adjusting film 21 and the material of the phaseadjusting film 22 makes it possible to improve the selection ratio atetching for processing the phase shift film.

According to the second embodiment, a single layered structure of thetransmittance adjusting film 21 is formed on the portion of thetransparent substrate 20 in the peripheral portion formation region, sothat the transmittance of the peripheral portion is lower than that ofthe transparent substrate 20, and therefore the peripheral portionserves as a transmittance adjusting portion. That is to say, thetransmittance of the peripheral portion can be adjusted to a desiredvalue by the transmittance adjusting film 21. Therefore, it is avoidedthat the transmittance of the peripheral portion is the highest on thephotomask, so that the degree of miniaturization required for theperipheral portion can be reduced. In other words, the problem that theupper limit of the size of the peripheral portion, i.e., the opening inthe outline enhancement mask is small, which makes it difficult toproduce a photomask, can be prevented.

According to the second embodiment, after the transmittance adjustingfilm 21 and the phase adjusting film 22 are formed sequentially on thetransparent substrate 20, the phase adjusting film 22, the transmittanceadjusting film 21 and the transparent substrate 20 are etchedselectively, and therefore a mask pattern with any shape can be easilyrealized that has the low transmittance phase shifter(semi-light-shielding portion) and the transmittance adjusting portion(peripheral portion), and a light-transmitting portion with any shapecan be easily realized that serves as the high transmittance phaseshifter.

According to the second embodiment, an opening (transmittance adjustingportion) having any shape can be formed by processing the phaseadjusting film 22 constituting the low transmittance phase shifter, sothat as the layout of the outline enhancement mask, not only the typeshown in FIGS. 18B and 18C, that is, the type shown in FIG. 9B, but alsoall the types shown in FIGS. 9A to 9F, for example, can be realized.

In the second embodiment, it is preferable that the transmittance of thephase shift film having a multilayered structure of the transmittanceadjusting film 21 and the phase adjusting film 22 is 6% or more and 15%or less. Thus, the contrast enhancement effect can be obtained reliablywhile preventing a reduction in thickness of the resist film in patternformation.

In the second embodiment, the description is based on the use of thepositive resist process, but the negative resist process can be used,instead of the positive resist process. In this case, in either one ofthe processes, as the exposure light source, the i line (wavelength 365nm), KrF excimer laser light (wavelength 248 nm), ArF excimer laserlight (wavelength 193 nm), or F₂ excimer laser light (wavelength 157 nm)can be used, for example.

In the second embodiment, it is preferable that the transmittanceadjusting film 21 is made of a single layered film that generates aphase difference of (−30+360×n) degrees or more and (30+360×n) degreesor less (where n=an integer) between this film and the transparentsubstrate 20. By doing this, photomask process can be performed easilyonly by preparing a mask blank for a regular half-tone phase-shiftingmask in which a transmittance adjusting film made of a single layeredthin film and a phase shift film made of a phase adjusting film areformed on a transparent substrate, and etching each of the phaseadjusting film, the transmittance adjusting film and the transparentsubstrate. In this case, as the transmittance adjusting film 21, forexample, a metal thin film is used, the transmittance adjusting film 21can be utilized as an etching mask having a high selection ratio withrespect to the transparent substrate 20 made of quartz or the like insubstrate etching for forming the dug portion 20 a in the transparentsubstrate 20, which is advantageous.

First Variation of the Second Embodiment

Hereinafter, a photomask of a first variation of the second embodimentand a method for producing the photomask will be described withreference to the accompanying drawings.

The first variation of the second embodiment is different from thesecond embodiment in the following aspects. In the second embodiment,the outline enhancement mask having a layout in which the hightransmittance phase shifter (light-transmitting portion) and the opening(transmittance adjusting portion) are adjacent as shown, for example, inFIGS. 9A to 9C is described. In the first variation of the secondembodiment, the outline enhancement mask having a layout in which thehigh transmittance phase shifter and the opening are apart as shown, forexample, in FIGS. 9D to 9F is described.

FIGS. 21A to 21E are cross-sectional views showing the processes of amethod producing a photomask of the first variation example of thesecond embodiment. FIG. 21F is a plan view corresponding to thecross-sectional view of FIG. 21C, and FIG. 21G is a plan viewcorresponding to the cross-sectional view of FIG. 21E.

First, as shown in FIG. 21A, a transmittance adjusting film 21 having alower transmittance with respect to exposure light than that of atransparent substrate 20 and a phase adjusting film 22 are formedsequentially on the transparent substrate 20 made of a material havinglight-transmitting properties with respect to exposure light, such asquartz. The phase adjusting film 22 generates a phase difference of(150+360×n) degrees or more and (210+360×n) degrees or less (where n=aninteger) with respect to exposure light between this film and amultilayered structure of the transparent substrate 20 and thetransmittance adjusting film 21 (i.e., transmittance adjusting portion).The phase shift film made of a multilayered structure of thetransmittance adjusting film 21 and the phase adjusting film 22 is asemi-light-shielding portion having a predetermined transmittance (6% to15%) with respect to exposure light.

Next, as shown in FIG. 21B, a first resist pattern 23 that covers a lowtransmittance phase shifter (semi-light-shielding portion) formationregion is formed on the transparent substrate 20. That is, a firstresist pattern 23 having a removed portion in each of a hightransmittance phase shifter (light-transmitting portion) formationregion and a transmittance adjusting portion (peripheral portion)formation region is formed. In this example, the transmittance adjustingportion formation region and the high transmittance phase shifterformation region are apart. In other words, the first resist pattern 23is interposed between the transmittance adjusting portion formationregion and the high transmittance phase shifter formation region.Thereafter, the phase adjusting film 22 is etched with the first resistpattern 23 as a mask to pattern the phase adjusting film 22. Then, thefirst resist pattern 23 is removed. Thus, as shown in FIGS. 21C and 21F,the portion corresponding to each of the high transmittance phaseshifter formation region and the transmittance adjusting portionformation region in the phase adjusting film 22 is removed.

Next, as shown in FIG. 21D, a second resist pattern 24 that covers thelow transmittance phase shifter formation region including thetransmittance adjusting portion formation region and that has a removedportion in the high transmittance phase shifter formation region isformed on the transparent substrate 20. Thereafter, the transmittanceadjusting film 21 and the transparent substrate 20 are etchedsequentially with the second resist pattern 24 and the patterned phaseadjusting film 22 as masks. Then, the second resist pattern 24 isremoved. Thus, as shown in FIGS. 21E and 21G, the portion correspondingto the high transmittance phase shifter formation region in thetransmittance adjusting film 21 is removed. Furthermore, a dug portion20 a that generates a phase inversion of 180 degrees (more specifically,(150+360×n) degrees or more and (210+360×n) degrees or less (where n isan integer)) is formed in the portion corresponding to the hightransmittance phase shifter formation region in the transparentsubstrate 20, and thus the photomask of the first variation of thesecond embodiment is completed.

According to the first variation of the second embodiment, the followingadvantages can be obtained, in addition to those of the secondembodiment. Since the patterned phase adjusting film 22 is used as amask for etching the transparent substrate 20 in a self-alignmentmanner, photomask process can be performed precisely.

Second Variation of the Second Embodiment

Hereinafter, a photomask of a second variation of the second embodimentand a method for producing the photomask will be described withreference to the accompanying drawings.

The second variation of the second embodiment is different from thesecond embodiment in the following aspects. In the second embodiment,the outline enhancement mask having a layout in which the hightransmittance phase shifter (light-transmitting portion) and the opening(transmittance adjusting portion) are adjacent as shown, for example, inFIGS. 9A to 9C is described. In the second variation of the secondembodiment as well as the first variation of the second embodiment, theoutline enhancement mask having a layout in which the high transmittancephase shifter and the transmittance adjusting portion are apart asshown, for example, in FIGS. 9D to 9F is described.

FIGS. 22A to 22E are cross-sectional views showing the processes of amethod producing a photomask of the second variation example of thesecond embodiment. FIG. 22F is a plan view corresponding to thecross-sectional view of FIG. 22C, and FIG. 22G is a plan viewcorresponding to the cross-sectional view of FIG. 22E.

First, as shown in FIG. 22A, a transmittance adjusting film 21 having alower transmittance with respect to exposure light than that of atransparent substrate 20 and a phase adjusting film 22 are formedsequentially on the transparent substrate 20 made of a material havinglight-transmitting properties with respect to exposure light, such asquartz. The phase adjusting film 22 generates a phase difference of(150+360×n) degrees or more and (210+360×n) degrees or less (where n=aninteger) with respect to exposure light between this film and amultilayered structure of the transparent substrate 20 and thetransmittance adjusting film 21. (i.e., transmittance adjustingportion). The phase shift film made of a multilayered structure of thetransmittance adjusting film 21 and the phase adjusting film 22 is asemi-light-shielding portion having a predetermined transmittance (6% to15%) with respect to exposure light.

Next, as shown in FIG. 22B, a first resist pattern 23 that covers aregion a low transmittance phase shifter (semi-light-shielding portion)formation region and a high transmittance phase shifter(light-transmitting portion) formation region is formed on thetransparent substrate 20. That is, a first resist pattern 23 having aremoved portion in a transmittance adjusting portion (peripheralportion) formation region is formed. Thereafter, the phase adjustingfilm 22 is etched with the first resist pattern 23 as a mask to patternthe phase adjusting film 22. Then, the first resist pattern 23 isremoved. Thus, as shown in FIGS. 22C and 22F, the portion correspondingto the transmittance adjusting portion formation region in the phaseadjusting film 22 is removed.

Next, as shown in FIG. 22D, a second resist pattern 24 that covers thelow transmittance phase shifter formation region and the transmittanceadjusting portion formation region is formed on the transparentsubstrate 20. That is to say, a second resist pattern 24 that has aremoved portion in the high transmittance phase shifter formation regionis formed on the transparent substrate 20. Thereafter, the phaseadjusting film 22, the transmittance adjusting film 21 and thetransparent substrate 20 are etched sequentially with the second resistpattern 24 as a mask. Then, the second resist pattern 24 is removed.Thus, as shown in FIGS. 22E and 22G, the portion corresponding to thehigh transmittance phase shifter formation region in each of thetransmittance adjusting film 21 and the phase adjusting film 22 isremoved. Furthermore, a dug portion 20 a that generates a phaseinversion of 180 degrees (more specifically, (150+360×n) degrees or moreand (210+360×n) degrees or less (where n is an integer)) between thisportion and a multilayered structure of the transparent substrate 20 andthe transmittance adjusting film 21 (i.e., transmittance adjustingportion) is formed in the portion corresponding to the hightransmittance phase shifter formation region in the transparentsubstrate 20, and thus the photomask of the second variation of thesecond embodiment is completed.

According to the second variation of the second embodiment, thefollowing advantages can be obtained, in addition to those of the secondembodiment. In this variation, the process of removing the portioncorresponding to the transmittance adjusting portion formation region inthe phase adjusting film 22 (see FIG. 22C) and the process of removingthe portion corresponding to the high transmittance phase shifterformation region in the phase adjusting film 22 (see FIG. 22E) areperformed separately. Therefore, if the transmittance adjusting portionis apart from the high transmittance phase shifter with a smalldistance, in other words, if the phase adjusting film 22 having a smallwidth is left between the transmittance adjusting portion and the hightransmittance phase shifter, the margin for photomask process becomeslarge.

In the second variation of the second embodiment, before performing theprocess of removing the portion corresponding to the transmittanceadjusting portion formation region in the phase adjusting film 22, theprocess of removing the portion corresponding to the high transmittancephase shifter formation region in the phase adjusting film 22 (includingthe process of removing the portion corresponding to the hightransmittance phase shifter formation region in the transmittanceadjusting film 21 and forming a dug portion 20 a in the transparentsubstrate 20) may be performed.

THIRD EMBODIMENT

Hereinafter, a photomask according to a third embodiment of the presentinvention, a method for producing the photomask and a method for forminga pattern using the photomask will be described with reference to theaccompanying drawings. The photomask of the third embodiment is aphotomask of a reduction projection exposure system to realize theabove-described outline enhancement method.

FIG. 23A shows an example of a desired pattern to be formed with thephotomask of the third embodiment.

In this embodiment, when describing pattern formation, the descriptionis based on the positive resist process, unless otherwise specified.That is to say, the description is based on the assumption that anexposed portion of the resist film is removed. On the other hand, when anegative resist process is assumed to be used, the description istotally the same as in the case of the positive resist process, exceptthat the exposed portion of the resist film becomes a resist pattern. Inthis embodiment, the transmittance is expressed by an effectivetransmittance when the transmittance of the transparent substrate istaken as 100%, unless otherwise specified.

FIG. 23B is a plan view of the photomask of the third embodiment, morespecifically, a photomask for forming the desired pattern shown in FIG.23A. As shown in FIG. 23B, high transmittance phase shifters(light-transmitting portions) are provided so as to correspond toresist-removed portions in the desired pattern. Furthermore, a lowtransmittance phase shifter (semi-light-shielding portion) having a lowtransmittance (about 6 to 15%) that does not allow the resist film to beexposed is used as the light-shielding mask pattern surrounding the hightransmittance phase shifter, instead of the complete light-shieldingportion that completely shields exposure light. Openings (peripheralportion) having a small width that is not provided with the lowtransmittance phase shifter are provided in the vicinity of the hightransmittance phase shifters. The high transmittance phase shifter andthe low transmittance phase shifter transmit exposure light in the samephase, whereas the openings transmits exposure light in a phase oppositeto the high transmittance phase shifter and the low transmittance phaseshifter.

In the third embodiment, for example, as shown in FIG. 9B, the openingsare arranged in such a manner that the sides of the openings are incontact with the corresponding sides of the rectangular hightransmittance phase shifter in a region having a predetermine size orless from each side of the rectangular high transmittance phase shifter.

FIG. 23C is a cross-sectional view taken along line AA′ in FIG. 23B,that is, a cross-sectional view of the photomask of the thirdembodiment. As shown in FIG. 23C, the photomask shown in FIG. 23B isrealized in the following manner. A phase adjusting film 31 and asemi-light-shielding film (transmittance adjusting film) 32 having alower transmittance with respect to exposure light than that of thetransparent substrate 30 are formed sequentially on the portion of thetransparent substrate 30 in the low transmittance phase shifterformation region. The phase adjusting film 31 generates a phasedifference of 180 degrees (more specifically (150+360×n) degrees or moreand (210+360×n) degrees or less (where n is an integer)) with respect toexposure light between this film and the transparent substrate 30(opening). Thus, a phase shift film having a multilayered structure ofthe phase adjusting film 31 and the transmittance adjusting film 32 andhaving a low transmittance (about 6% to 15%) that does not allow theresist film to be exposed is formed, and thus a semi-light-shieldingportion serving as the low transmittance phase shifter is formed. Inthis embodiment, it is assumed that the transmittance adjusting film 32transmits light at a low transmittance and a phase change of light bythe thickness of the transmittance adjusting film 32 is only small.Furthermore, a single layered structure of the phase adjusting film 31is formed on the portion of the transparent substrate 30 in thelight-transmitting portion formation region, and thus alight-transmitting portion serving as the high transmittance phaseshifter is formed. Therefore, the high transmittance phase shifter(light-transmitting portion) and the low transmittance phase shifter(semi-light-shielding portion) made of the multilayered structure of thephase adjusting film 31 and the transmittance adjusting film 32 (thephase shift film) sandwich the peripheral portion, that is, the opening(the surface of the transparent substrate 30 is exposed) that is notprovided with the phase adjusting film 31, and thus an outlineenhancement mask is realized. As the phase adjusting film 31, an oxidefilm such as SiO₂ film can be used. As the phase adjusting film 32, athin film (having a thickness of 30 nm or less) made of a metal such asZr, Cr, Ta, Mo or Ti or a thin film (having a thickness of 30 nm orless) made of a metal alloy such as a Ta—Cr alloy, a Zr—Si alloy, aMo—Si alloy or a Ti—Si alloy can be used. However, in order to enhancecontrast by the outline enhancement method, it is necessary to limit thewidth of the opening to a predetermined size or less.

Next, a method for forming a pattern using the photomask of the thirdembodiment will be described.

FIGS. 24A to 24D are cross-sectional views showing the processes of amethod forming patterns with the photomask of the third embodiment.

First, as shown in FIG. 24A, after a film 301 to be processed such as ametal film or an insulating film is formed on a substrate 300, as shownin FIG. 24B, a positive resist film 302 is formed on the film to beprocessed 301.

Next, as shown in FIG. 24C, the photomask of the third embodimentincluding the low transmittance phase shifter made of the multilayeredstructure (phase shift film) of the phase adjusting film 31 and thetransmittance adjusting film 32 and a light-transmitting portionfunctioning as a high transmittance phase shifter by the single layeredstructure of the phase adjusting film 31 is irradiated with exposurelight 303 with an oblique incident exposure light source to expose theresist film 302 with transmitted light 304 transmitted through thephotomask. In this case, as the mask pattern, the low transmittancephase shifter (semi-light-shielding portion) is used, so that the entireresist film 302 is exposed with weak energy. However, as shown in FIG.24C, only a latent image portion 302 a of the resist film 302corresponding to the light-transmitting portion in the photomask isirradiated with exposure energy that is sufficient to dissolve theresist film 302 in a developing process.

Next, the latent image portion 302 a is removed by performingdevelopment with respect to the resist film 302, so that as shown inFIG. 24D, a resist pattern 305 is formed. In this case, in the exposureprocess shown in FIG. 24C, light around the light-transmitting portionis canceled, so that a portion corresponding to the opening portion(peripheral portion) in the resist film 302 is substantially notirradiated with exposure energy. Therefore, the contrast in the lightintensity distribution between the light transmitted through thelight-transmitting portion and the light transmitted through theperipheral portion, in other words; the contrast in the light intensitydistribution between the light with which the latent image portion 302 ais irradiated and the light with which the periphery of the latentportion 302 a is irradiated can be enhanced. Therefore, the energydistribution in the latent portion 302 a is changed sharply, so that aresist pattern 305 having a sharp shape can be formed.

Next, a method for producing a photomask of the third embodiment will bedescribed with reference to the drawings.

FIGS. 25A to 25E are cross-sectional views showing the processes of amethod producing the photomask of the third embodiment. FIG. 25F is aplan view corresponding to the cross-sectional view of FIG. 25C, andFIG. 25G is a plan view corresponding to the cross-sectional view ofFIG. 25E.

First, as shown in FIG. 25A, a phase adjusting film 31 and atransmittance adjusting film 32 having a transmittance lower than thatof the transparent substrate 30 with respect to exposure light areformed sequentially on a transparent substrate 30 made of a materialhaving light-transmitting properties with respect to exposure light,such as quartz. The phase adjusting film 31 generates a phase differenceof (150+360×n) degrees or more and (210+360×n) degrees or less (wheren=an integer) with respect to exposure light between this film and thetransparent substrate 30 (opening). The phase shift film having amultilayered structure including the phase adjusting film 31 and thetransmittance adjusting film 32 constitutes a semi-light-shieldingportion having a predetermined transmittance (e.g., 6 to 15%) withrespect to exposure light. In this embodiment, for example, alight-shielding film (chromium film or the like used as alight-shielding film of a regular photomask) that has come to have a lowtransmittance by making the film thickness small can be used as thetransmittance adjusting film 32.

Next, as shown in FIG. 25B, a first resist pattern 33 that covers thelow transmittance phase shifter (semi-light-shielding portion) formationregion and the high transmittance phase shifter (light-transmittingportion) formation region is formed on the transparent substrate 30.That is, a first resist pattern 33 having a removed portion in theopening (peripheral portion) formation region is formed on thetransparent substrate 30. Thereafter, the transmittance adjusting film32 and the phase adjusting film 31 are etched with the first resistpattern 33 as a mask. Then, the first resist pattern 33 is removed.Thus, as shown in FIGS. 25C and 25F, the portion corresponding to theopening formation region in the multilayered structure (phase shiftfilm) of the phase adjusting film 31 and the transmittance adjustingfilm 32 is removed.

Next, as shown in FIG. 25D, a second resist pattern 34 that covers atleast the low transmittance phase shifter formation region and has aremoved portion in the high transmittance phase shifter formation regionis formed on the transparent substrate 30. Thereafter, the transmittanceadjusting film 32 is etched with the second resist pattern 34 as a mask.Then, the second resist pattern 34 is removed. Thus, as shown in FIGS.25E and 25G, the portion corresponding to the high transmittance phaseshifter formation region in the transmittance adjusting film 32 isremoved, and thus the photomask of the third embodiment is completed.That is to say, the photomask of the third embodiment having a planestructure of the outline enhancement mask can be easily formed by, as amask blank, preparing a transparent substrate in which a phase adjustingfilm that generates a phase inversion of 180 degrees and a thinlight-shielding film (transmittance adjusting film) are depositedsequentially, and then performing etching with respect to thelight-shielding film and the phase adjusting film sequentially.

As described above, according to the third embodiment, the phase shiftfilm (a multilayered structure of the phase adjusting film 31 and thetransmittance adjusting film 32) that transmits exposure light at a lowtransmittance with a phase inversion is formed on the portion of thetransparent substrate 30 in the low transmittance phase shifter(semi-light-shielding portion) formation region. Furthermore, a singlelayered structure of the phase adjusting film 31 is formed on theportion of the transparent substrate 30 in the light-transmittingportion formation region so that a light-transmitting portion is formed.Therefore, the opening that is not provided with the phase shift film,that is, the peripheral portion that transmits exposure light in thephase opposite to that of the light-transmitting portion is sandwichedby the light-transmitting portion that serves as the high transmittancephase shifter by the single layered structure of the phase adjustingfilm 31 and the low transmittance phase shifter that transmits exposurelight in the same phase as that of the light-transmitting portion andmade of the phase shift film. As a result, the contrast in the lightintensity distribution between the light-transmitting portion and theperipheral portion can be enhanced by mutual interference between thelight transmitted through the peripheral portion and the lighttransmitted through the light-transmitting portion. This contrastenhancement effect also can be obtained when a fine isolatedresist-removed portion (i.e., a fine isolated space pattern) is formedwith oblique incident exposure, for example, in the positive resistprocess. That is to say, a combination of the photomask of thisembodiment and oblique incident exposure can miniaturize isolated spacepatterns and isolated line patterns or dense patterns at the same time.

According to the third embodiment, the phase shift film serving as a lowtransmittance phase shifter has a multilayered structure of the phaseadjusting film 31 having a high transmittance and the transmittanceadjusting film 32 having a low transmittance. Therefore, a combinationof a desired phase change and a desired transmittance can be selectedarbitrarily for the phase shift film. A combination of the material ofthe phase adjusting film 31 and the material of the transmittanceadjusting film 32 makes it possible to improve the selection ratio atetching for processing the phase shift film.

According to the third embodiment, after the phase adjusting film 31 andthe transmittance adjusting film 32 are formed sequentially on thetransparent substrate 30, the transmittance adjusting film 32 and thephase adjusting film 31 are etched selectively, and therefore a maskpattern with any shape can be easily realized that has the lowtransmittance phase shifter (semi-light-shielding portion) and theopening (peripheral portion), and a light-transmitting portion with anyshape can be easily realized that serves as the high transmittance phaseshifter.

According to the third embodiment, an opening with any shape can beformed by processing the multilayered structure (phase shift film) ofthe phase adjusting film 31 and the transmittance adjusting film 32constituting the low transmittance phase shifter, so that as the layoutof the outline enhancement mask, not only the type shown in FIGS. 23Band 23C, that is, the type shown in FIG. 9B, but also all the typesshown in FIGS. 9A to 9F, for example, can be realized.

In the third embodiment, the transmittance adjusting film 32 is made ofa thin light-shielding film, that is, a single layered film thatgenerates a phase difference of (−30+360×n) degrees or more and(30+360×n) degrees or less (where n=an integer) between this film and amultilayered structure of the transparent substrate 30 and the phaseadjusting film 31. Accordingly, the following advantages can beprovided. The photomask process can be performed easily only bypreparing a mask blank for a half-tone phase-shifting mask in which aphase shift film including a phase adjusting film, which is a lowerlayer, and a transmittance adjusting film, which is an upper layer, isformed on a transparent substrate, and etching each of the phaseadjusting film and the transmittance adjusting film. In other words,there is an advantage that a conventional technique can be used in thephotomask production. Furthermore, since the transmittance adjustingfilm is a thin light-shielding film, the structure of a mask blank to beprepared is very simple.

Hereinafter, the results of examination with simulations of an influenceof a phase change (a phase difference caused between the hightransmittance phase shifter and the low transmittance phase shifter) dueto the use of the thin light-shielding film as the transmittanceadjusting film 32 on the pattern formation will be described withreference to FIGS. 26A to 26C. The simulation conditions are such thatthe wavelength λ of the exposure light is 0.193 μm (ArF light source),the numerical aperture NA of the projection optical system of theexposure apparatus is 0.6, and annular illumination is used.

FIG. 26A shows a plan view of an outline enhancement mask used in thesimulations. As shown in FIG. 26A, the width of the high transmittancephase shifter (light-transmitting portion) and the opening (peripheralportion) is 200 nm and 50 nm, respectively. The transmittance of thehigh transmittance phase shifter, the opening and the low transmittancephase shifter (semi-light-shielding portion) is 100%, 100% and 7.5%,respectively. The high transmittance phase shifter generates a phasedifference of 180 degrees between this portion and the opening, and thelow transmittance phase shifter generates a phase difference of 180 to150 degrees between this portion and the opening.

FIG. 26B shows the simulation results of the light intensitydistribution corresponding to line AA′ when exposure is performed withrespect to the outline enhancement mask shown in FIG. 26A in such amanner that phase differences of 180 degrees, 170 degrees, 160 degreesand 150 degrees are generated by the low transmittance phase shifterbetween this shifter and the opening. As shown in FIG. 26B, if the phasedifference between the high transmittance phase shifter and the lowtransmittance phase shifter is not more than 30 degree or so, thecontrast in the light intensity distribution is not substantiallyaffected.

FIG. 26C shows the simulation results of the focus dependence of thesize of the finished pattern (CD: Critical Dimension) when exposure isperformed with respect to the outline enhancement mask shown in FIG. 26Ain such a manner that phase differences of 180 degrees, 170 degrees, 160degrees and 150 degrees are generated by the low transmittance phaseshifter between this shifter and the opening. As shown in FIG. 26C, ifthe phase difference between the low transmittance phase shifter and thehigh transmittance phase shifter is changed, the best focus position inwhich the CD is the peak is changed. However, even if the phasedifference is changed, the unlikelihood of CD change with respect to thefocus variation, that is, the depth of focus is substantially notchanged. No problem is caused in pattern formation, even if the bestfocus positions are varied in the same manner at all portions on thephotomask. Only the depth of focus is an issue in pattern formation.That is to say, if the phase difference between the low transmittancephase shifter and the high transmittance phase shifter is up to about 30degrees, there is not problem in terms of the focus characteristics.

Therefore, in this embodiment, when a thin light-shielding film is usedas the transmittance adjusting film 32, the outline enhancement mask ina strict sense (the phase difference between the low transmittance phaseshifter and the high transmittance phase shifter is 0 degree) cannot berealized, but if the phase difference that is caused by a thin film isabout 30 degrees or less, the effect of the outline enhancement methodis not lost. More specifically, when Ta, Cr or alloys containing Ta orCr or the like is used as the material of the light-shielding film, thethickness of the light-shielding film that generates a phase differenceof about 30 degrees between this film and the high transmittance phaseshifter (light-transmitting portion) with respect to light from an ArFlight source is approximately 30 nm or more. This thickness issufficient to realize a transmittance of 10% or less.

In the third embodiment, it is preferable that the transmittance of thephase shift film having a multilayered structure of the phase adjustingfilm 31 and the transmittance adjusting film 32 is 6% or more and 15% orless. Thus, the contrast enhancement effect can be obtained reliablywhile preventing a reduction in thickness of the resist film in patternformation.

In the third embodiment, the description is based on the use of thepositive resist process, but the negative resist process can be used,instead of the positive resist process. In this case, in either one ofthe processes, as the exposure light source, the i line (wavelength 365nm), KrF excimer laser light (wavelength 248 nm), ArF excimer laserlight (wavelength 193 nm), or F₂ excimer laser light (wavelength 157 nm)can be used, for example.

In the third embodiment, the outline enhancement mask having a layout inwhich the high transmittance phase shifter is adjacent to the opening,for example, as shown in FIGS. 9A to 9C has been described, but theoutline enhancement mask can have a layout in which the hightransmittance phase shifter is apart from the opening, for example, asshown in FIGS. 9D to 9F.

Furthermore, in the third embodiment, the phase difference between thehigh transmittance phase shifter and the low transmittance phase shiftercan be substantially zero by depositing another phase adjusting film onthe transmittance adjusting film 32.

In the first to third embodiments, it is assumed that all the portionsexcept the opening (which may be the peripheral portion or thetransmittance adjusting portion) and the high transmittancephase-shifter (light-transmitting portion) are made of a lowtransmittance phase shifter (semi-light-shielding portion). However, theportion in the photomask that is apart from each of the opening and thehigh transmittance phase shifter by a sufficient distance, that is, adistance (=2×λ/NA (λ is the wavelength of exposure light, and NA is thenumerical aperture of a reduction projection optical system of anexposure apparatus)) that allows an influence of optical interferenceeffects from each of the opening and the high transmittance phaseshifter to be ignored may be a complete light-shielding portion.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof. The embodiments disclosedin this application are to be considered in all respects as illustrativeand not limiting. The scope of the invention is indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

1. A photomask on a transparent substrate comprising: a first phaseshifter having a semi-light-shielding property with respect to exposurelight; a second phase shifter surrounded by the first phase shifter andhaving a light-transmitting property with respect to exposure light; andan opening surrounded by the first phase shifter and positioned in aperiphery of the second phase shifter, wherein the first phase shifterand the second phase shifter transmit the exposure light in a samephase, and the opening transmits the exposure light in a phase oppositeto that of the first phase shifter and the second phase shifter.
 2. Thephotomask according to claim 1, wherein a first phase shift film isformed on the transparent substrate, as the first phase shifter.
 3. Thephotomask according to claim 1, wherein a substrate-dug portion isformed by digging the transparent substrate, as the second phaseshifter.
 4. The photomask according to claim 1, wherein a second phaseshift film is formed on the transparent substrate, as the second phaseshifter.
 5. The photomask according to claim 1, wherein in the opening,the first phase shifter and the second phase shifter are not formed. 6.The photomask according to claim 1, wherein a transmittance with respectto the exposure light of the first phase shifter is 6% or more and 15%or less.
 7. The photomask according to claim 1, wherein a transmittancewith respect to the exposure light of the second phase shifter is 60% ormore.
 8. The photomask according to claim 1, wherein a first phase shiftfilm is formed on the transparent substrate, as the first phase shifter,and a substrate-dug portion is formed by digging the transparentsubstrate, as the second phase shifter.
 9. The photomask according toclaim 1, wherein a transmittance with respect to the exposure light ofthe first phase shifter is lower than a transmittance with respect tothe exposure light of the second phase shifter.
 10. The photomaskaccording to claim 1, wherein a transmittance with respect to theexposure light of the first phase shifter is 6% or more and 15% or less,and a transmittance with respect to the exposure light of the secondphase shifter is 60% or more.
 11. The photomask according to claim 1,wherein the second phase shifter has a square shape or rectangularshape, and the opening comprises a ring-shaped region in contact withthe peripheral portion of the second phase shifter.
 12. The photomaskaccording to claim 1, wherein the second phase shifter has a squareshape or rectangular shape, and the opening comprises a plurality ofrectangular regions each of which is in contact with each side of thesecond phase shifter.
 13. The photomask according to claim 1, whereinthe second phase shifter has a square shape or rectangular shape, theopening comprises a ring-shaped region, and a part of the first phaseshifter having a ring shape is present between the opening and thesecond phase shifter.
 14. The photomask according to claim 1, whereinthe second phase shifter has a square shape or rectangular shape, theopening comprises a plurality of rectangular regions each facing with aside of the second phase shifter, and a part of the first phase shifterhaving a ring shape is present between the opening and the second phaseshifter.
 15. The photomask according to claim 1, wherein the exposurelight transmitted through the first phase shifter and the exposure lighttransmitted through the opening have a phase difference of (150+360×n)degrees or more and (210+360×n) degrees or less (where n=an integer),and the exposure light transmitted through the second phase shifter andthe exposure light transmitted through the opening have a phasedifference of (150+360×n) degrees or more and (210+360×n) degrees orless (where n=an integer).
 16. The photomask according to claim 8,wherein the first phase shift film is a metal-containing oxide film. 17.The photomask according to claim 8, wherein the first phase shift filmincludes a transmittance adjusting film having a transmittance lowerthan that of the transparent substrate with respect to the exposurelight and a phase adjusting film that is formed on the transmittanceadjusting film, and transmits the exposure light in a phase opposite tothat of the opening.
 18. The photomask according to claim 17, whereinthe opening is disposed in a position apart from the second phaseshifter by a predetermined distance, and only the transmittanceadjusting film of the first phase shift film is formed between theopening and the second phase shifter.
 19. The photomask according toclaim 1, wherein the opening is disposed so as to be in contact with thesecond phase shifter.
 20. The photomask according to claim 1, whereinthe opening is disposed apart from the second phase shifter by apredetermined distance.